A new anxiety is filtering through the NFL's officiating ranks. The first training camp opens in 10 days. Preseason games are less than a month away. There has been unprecedented turnover among referees, including two unexpected retirements last month. And because of late approval from owners, hardly any of the league's 121 officials have been briefed in detail on a series of rule changes that will require fundamental changes in the way they administrate games.

That confluence will make for a hectic three days this weekend in Dallas, where senior vice president of officiating Al Riveron will lead the league's annual officiating clinic. Some of the apprehension will be soothed there, but even informed observers worry that the NFL is asking too much of the department, given the circumstances.

In short, the NFL will navigate the biggest collection of rule changes in recent memory this year with one of its least experienced groups of officials ever.

"To be quite honest," former NFL officiating supervisor Jim Daopoulos said, "I have a concern about the level of officiating because of the inexperience out there and with the new rule changes. You've got so many things that the young officials are going to have to learn and the veteran officials are going to have to learn.

"It's going to be a very interesting year, and I think Al Riveron has really got his hands full. It's going to be really tough. You think about the experience he's lost this year and trying to replace these guys, it's going to make for some tough Sundays watching football."

The NFL did not respond to a request for comment from Riveron, who is scheduled to speak with reporters on Friday in Dallas. Officials themselves are barred from speaking with reporters. But former referee Terry McAulay, who retired last month to join NBC Sports, said the concern is real.

"These are probably the most significant rule changes I've seen in my career in one season," McAulay told ESPN.

Unprecedented turnover

Four referees retired this spring, the most in one offseason in league history, according to research by Football Zebras. The NFL was expecting two: Ed Hochuli and Jeff Triplette. Both spoke privately about it for months. They were replaced quickly by Shawn Hochuli and Alex Kemp; each is the son of a referee -- Ed Hochuli and Stan Kemp -- and have long been earmarked as future referees.

According to sources, however, NBC approached multiple NFL referees about a job in late spring that its executives felt was essential to broadcasting football games: a rules expert. Clete Blakeman turned down an offer. McAulay, 58, accepted. CBS had reached a similar conclusion. Within days, it hired Gene Steratore.

Suddenly, two of the NFL's most respected referees were gone in McAulay and Steratore. There is no evidence to suggest either was upset or otherwise fleeing an untenable situation. If anything, McAulay said, it was a perfect storm of events.

"I had a minimum of another five years in the NFL left in me," McAulay said. "It wasn't something I was actively seeking out. ... I had 20 years in the NFL, but I've also had a pretty good career. There are only 17 referees, and there are only four spots for [a rules analyst]. It was just too awesome of an opportunity to pass up. My wife and I discussed it for a while, the pros and cons, but ultimately it was the right thing to do at the right time.

"If I didn't do it, somebody else would have. That person could have been there for 10 years and then the opportunity is absolutely gone. Everything just came together."

CBS declined a request to interview Steratore.

The NFL can take part of the blame for the departures, having created a set of rules that can no longer be navigated by non-experts in real time. Each network that broadcasts NFL games now has at least one former referee or league executive on staff. (Fox Sports employs two former officiating chiefs, Mike Pereira and Dean Blandino. ESPN, according to a source, has hired Triplette to replace Gerry Austin on Monday Night Football.)

Fred Gaudelli, executive producer of NBC's Sunday Night Football, said he considered approaching Blandino in recent years but didn't think he would leave the NFL. This spring, Gaudelli was determined to get an expert's voice on air alongside announcer Al Michaels and analyst Cris Collinsworth. McAulay will be on site for Sunday night games as well as Notre Dame football broadcasts.

"This is my 29th year doing NFL games," Gaudelli said. "The rules seem to get more complicated and more nuanced every single year. ... It was getting to the point where it was unrealistic to think that Al and Cris could really understand every nuance and every aspect of the game."

The move was probably a net financial gain, according to Daopoulos. Experienced NFL referees earn around $200,000 annually. The terms of McAulay and Steratore's deals are unknown, but Daopoulos said his conversations left him convinced that they are "much better off financially working in the media right now -- much better off."

A television career is not without risk to McAulay or Steratore, who would face uncertain prospects if they wanted to return as NFL referees. CBS' failed experiment with former referee Mike Carey from 2014-15 demonstrated that rules expertise is not the only job requirement. Carey's struggles to foretell the outcome of replay reviews became a near-weekly event.

"I think with Mike Carey, they got into the prediction mode," Gaudelli said. "But I'm not sure that's the route you want to take all the time. I just think that, with CBS and Steratore, I'll be surprised if they don't have a different plan of attack than what they had with Mike Carey."

The NFL scrambled to promote Shawn Smith and Clay Martin, both of whom entered the NFL in 2015, as replacement referees.

There has been at least one immediate impact of the turnover, according to Daopoulos. The crews of new referees are usually stacked with highly experienced officials at other positions, but that approach was unrealistic this spring. Shawn Hochuli, in fact, has a rookie side judge (Chad Hill) on his crew.

"It's just a numbers thing," Daopoulos said, "but it's a situation that scares me an awful lot. That rookie needs mentoring, and I'm just not sure they have the personnel to mentor a new referee and a new official on the same crew. That's part of why I think Al Riveron has got a tremendous job in front of him to get through this situation and get through this year."

That job isn't likely to get easier in the coming years. Four current referees are in their mid-60s: Walt Coleman, Walt Anderson, Tony Corrente and Pete Morelli. Since 2014, the NFL has replaced eight referees. Assuming at least four more retire in the next two years, the NFL will have turned over 70 percent of its referees in a seven-year period.

From his vantage point, McAulay said he believes that the NFL has a strong enough pipeline to withstand the departures.

"But I have to say, 'I think,' because until somebody is in that position, you don't know for sure," he said. "You have confidence in what they've done, but once you step into that first Sunday night or really any game, things change and you have to wait to see how they handle the adversity and the additional scrutiny that falls on the referee."

Rule changes complicate matters

In March, NFL owners approved a new catch rule after years of heavy public criticism. That would have been the headline of many offseasons, but instead, it has largely been forgotten amid two unexpected and unusually hasty rule changes: (1) The new "helmet rule" makes lowering a head to initiate contact a 15-yard penalty, plus a possible ejection; and (2) fundamental changes to the kickoff created a new alignment that has never been seen on an NFL field before.

When officials gathered in New York for a mid-May clinic, their first of the 2018 season, details of both rules remained in flux. As a result, there were no substantive discussions about either. This weekend's clinic will mark the first formal explanation most officials will receive on the technical requirements of the new rules.

"These are probably the most significant rule changes I've seen in my career in one season." Terry McAulay, retired NFL referee

Although he is no longer a referee, McAulay said he plans to attend the clinic to find out for himself how the league expects officials to administer them during games.

"It is going to be interesting," he said.

The NFL has sought to downplay concerns about the breadth of the helmet rule, especially as it relates to interior line play. But officials don't know anything more beyond the generalities league officials have expressed publicly.

After all, players lower their helmets for various reasons on nearly every play. Can officials reasonably intuit when the helmet is lowered to initiate contact?

"It's pretty straightforward verbiage," McAulay said, "but the application of that verbiage is what's going to be crucial."

The kickoff changes, meanwhile, have generated more concern among officials than the helmet rule.

"It isn't getting as much press," McAulay said, "but the kickoff change, this may be the biggest change I've ever seen. People know so little about it. There are so many restrictions on what either side of the ball can do. ... I spoke with a special-teams coordinator who is excited about it, and I know we all feel it can be great for the game. But [officials] have to wait and see. There are a lot of intricate rules in terms of what [players] can and cannot do."

The rule, for example, eliminates two-man wedges performed by players lined up in the new three-man return position. But it still allows double-team blocks by the eight players lined up in the front. In other words, officials will have to know where a player was lined up before the kick to know if his block is legal.

"This is going to be very difficult," Daopoulos said. "I don't think the officials know anything [of substance] about the new rules right now. They're going to try to learn them in three days while they're down in Dallas? When those guys go out there in the first couple of weeks, you're going to see a lot of quizzical looks on their faces."

None of this is to say that officials won't eventually absorb the new rules and administer them fairly, all while breaking in a record number of new referees. But for the moment, the hows and whys remain unanswered -- and the clock is ticking.