NFL history affords many instances of a cornerback joining a team and vaulting it to a championship. Herb Adderley, Mike Haynes and Deion Sanders all bolstered their already strong Hall of Fame cases by doing so, adding Super Bowl rings to their collections in the process.
This type of cornerback historical background is part of the reason the New England Patriots' signing of Darrelle Revis has been met with plenty of praise. However, that acquisition might not even be the most impactful offseason secondary signing in the NFL, as the metrics and game tape show that the New Orleans Saints' signing of free safety Jairus Byrd to a six-year, $54 million deal will have a bigger impact.
To illustrate this, let's start by estimating the statistical effect Revis could have in replacing Aqib Talib as New England's top cornerback.
Revis review
Coverage comparison: Revis versus Talib
Last season, Revis posted a 7.8 yards per attempt (YPA) mark for Tampa Bay, but that figure includes a 76-yard touchdown pass against New Orleans that was either a blown coverage by Revis or by safety Dashon Goldson. If Revis is given the benefit of the doubt on the play and not held responsible for this long gain, his YPA drops to 6.2.
A 6.2 YPA showing is well below the 7.5 YPA that serves as the rough median for cornerback performance in this metric, but Revis may be capable of even better numbers. Mark Dominik, who was the general manager in Tampa Bay last year and is now an ESPN analyst, said Revis "played a little cautious" through most of the 2013 season. His numbers could certainly improve in his second season after the ACL injury. For the sake of this discussion, let's assume he will be able to knock an additional yard off of his YPA mark and post a 5.2 YPA in 2014.
This compares quite well against Talib, who posted a 7.9 YPA on 65 targets last year. Using this YPA as a benchmark and using 65-75 targets as the estimated coverage workload for Revis in 2014, we can say that Revis should improve the Patriots coverage by 175.5 to 202.5 yards.
There is a chance that Revis' target volume will increase if the Patriots go with a type of scheme that funnels as many passes his way as possible. This is what the Jets did in 2009 when Revis had his season for the ages by leading all cornerbacks in targets (102) and YPA (3.6).
New England's odds of doing that might have been high until they acquired the services of former Seattle Seahawks cornerback Brandon Browner. Browner is a solid CB and had a lower YPA in 2012 than Richard Sherman (6.2 for Browner, 7.4 for Sherman). Funneling can place limits on what a defense can do coverage-wise and having two strong cornerbacks precludes the necessity of doing this. This is a major reason Seattle didn't funnel routes to Sherman and why the Patriots might not take that approach with Revis, which is why I'm keeping his target projection around the 65-75 level.
Ball hawking review: Revis versus Talib
In 2011 and 2013, his last two healthy seasons, Revis registered an interception or a near interception (defined as a ball that hit his catch frame but was not picked off) 10.8 percent and 8.9 percent of the time a pass was thrown his way. For the sake of this discussion, we can average those two numbers and say that he has a 10 percent pick chance percentage.
Talib has been a bit more inconsistent here, as he had a 14 percent pick chance rate in 2011, a 5.7 percent rate in 2012 and a 9.2 percent mark in 2013.
Revis also does a slightly better job in turning pick chances into actual interceptions. In 2011 and 2013, he had a total of 13 pick chances and caught six of them (46.1 percent conversion rate). Over the past three seasons, Talib had a total of 15 pick chances and converted only six of them into interceptions (40 percent conversion rate).
If those figures hold up for the 2014 season, the pick chance rate indicates Revis will likely have roughly as many pick chances as Talib would have notched. Revis does have a higher pick conversion rate but the 6.1 percent difference is not enough to say that he has a reasonable chance of turning more of the pick chances into interceptions.
Byrd review
Tracking free safety coverage performance can be a somewhat difficult task because when free safeties do their job well, quarterbacks will rarely throw the ball in their direction.
Having noted this, there are multiple pieces of evidence that show how Byrd could have a positive impact on the Saints secondary.
Medium pass-depth coverage
Last season, Buffalo (Byrd's 2013 team) ranked third in the league in Total QBR allowed on medium-depth passes (defined as aerials thrown 11-19 yards downfield). This was an area where New Orleans did a terrible job, as the Saints' 96.7 Total QBR ranked 29th.
One main reason for the performance difference is how these teams fared in medium-pass YPA. The Bills' 7.4 YPA here was the best in the NFL, while New Orleans' 10.0-yard mark ranked 17th. That was the main reason the Bills had a roughly 1-yard lead over the Saints in vertical YPA (9.54 for Buffalo, 10.59 for New Orleans). (Note: Vertical passes are defined as targets thrown 11 or more yards downfield)
If Byrd's addition can merely lower the Saints' medium YPA by a single yard, which is less than half of the medium YPA difference between these teams last season, it should knock off roughly 100-120 yards from the Saints' overall total (given the average number of medium-depth passes defenses typically face in a season).
Vertical pass ball hawking
Where Byrd really makes his biggest impact is in the area of ball hawking. Buffalo was one of the most dangerous teams to attempt a vertical pass down the middle against last year. According to ESPN Stats & Information, the Bills intercepted 12.9 percent of vertical aerials thrown between the numbers, a mark that ranked second in the league.
That wasn't a 2013 anomaly, either, as Buffalo has the leaguewide lead (9.3 percent) in interception rate on those types of throws over the past five years, a time frame that corresponds with Byrd's entry into their defensive backfield.
Not all of this can be attributed to Byrd, but he does have 22 interceptions in that time frame, a total that ranks second in the league and first among free safeties, so he deserves a good amount of credit.
By contrast, New Orleans ranked 22nd in that category in 2013 (5.2 percent) and tied for 29th over the past five seasons in that category (4.4 percent).
If Byrd is able to raise the Saints' total here by 4 percentage points, it should add three interceptions to their bottom line (using their 2013 total of 77 vertical passes thrown down the middle). That percentage level (9.2 percent) would be roughly equal to Buffalo's pick rate in that category over the past five seasons but would be well short of what the Bills posted in 2013. If Byrd can move New Orleans into the 10-13 percent interceptions-over-the-middle rate, the Saints could see their pick total increase by as many as six.
That number could go even higher due to the Saints being able to use this part of Byrd's game in a different manner than Buffalo did. The key here is if Kenny Vaccaro is able to return at full strength following his late-season injury. Vaccaro's varied skill set (Saints defensive coordinator Rob Ryan recently said he was the best safety in the league) should allow him to take over the box safety and slot coverage responsibilities that the Bills had Byrd assume on more than a few occasions. This would let Byrd to stick to playing center field and give him even more opportunities to showcase his interception skills and return the ball to a high-powered Saints offense.
Bottom line
Football statisticians have estimated that interceptions are worth about 45 yards. Using that as a benchmark, it can be said that Revis' bottom line impact is likely to be about 175.5 yards on the low end (175.5 yards on coverage improvement, zero yards on interceptions) and 202.5 yards on the high end (202.5 yards on coverage improvement, zero yards on interceptions).
Under that same system, Byrd's bottom-line impact is likely to be about 255 yards on the low end (120 coverage yards saved on medium passes, 135 yards on three interceptions) and 390 yards on the high end (120 coverage yards, 270 yards on six interceptions).
Both of these signings were championship-caliber coups, but Byrd's acquisition solves a bigger secondary issue for the Saints. That isn't to minimize what Revis will bring, as I expect the Patriots' defense to be even better this season with him taking Talib's place as New England's top corner. But the overall improvement Byrd will provide the Saints surpasses what Revis will provide the Pats, as his versatility and ball skills not only bolster the D but will lead to the creation of more scoring opportunities for Drew Brees and the offense. That makes him the most impactful secondary position free-agent signing of this offseason.