CLEVELAND -- With just over nine minutes still to go in the most ballyhooed Browns season opener since the days of Bernie Kosar -- heck, since maybe even Jim Brown -- the inconceivable occurred. Cleveland fans began filing out of FirstEnergy Stadium. The Dawg Pound, of all doggone places, thinned out into an orange swath of empty bleachers.

Even on this star-studded squad, featuring quarterback Baker Mayfield, receiver Odell Beckham Jr. and defensive end Myles Garrett, the Browns were always going to face adversity, at least at some point down the line. They were going to be doubted. They were -- as Mayfield would put it -- going to be tossed into the trash.

Virtually nobody, however, expected that to come before the first game of the season had even ended.

Not after Mayfield’s breakout rookie campaign through the air last year. Not after Cleveland then won the offseason by landing Beckham in a blockbuster deal with the New York Giants. Not after general manager John Dorsey had assembled an enviable roster that on paper seemed equipped to eradicate the NFL’s longest playoff and divisional title droughts.

But after the Tennessee Titans quashed all the hype in a 43-13 shellacking, the day of reckoning for Cleveland arrived in just one day.

“We lost our discipline and we lost our composure, but it’s one game and we’re going to be tested,” Cleveland coach Freddie Kitchens said. “You either take adversity and run together and run toward each other or you run away.”

The Browns spent most of Sunday running in the wrong direction. For the first time in 68 years, Cleveland committed 18 penalties for 182 yards, which derailed its own offensive drives while keeping several of Tennessee’s alive and rolling.

Kitchens and Mayfield correctly pointed out afterward that penalties were the biggest culprit behind the deflating defeat. Some, including a costly offensive pass interference on Beckham that wiped away a scoring opportunity just before halftime, were questionable. Others, such as left tackle Greg Robinson kicking Titans safety Kenny Vaccaro in the head, were blatant.

In either case, the penalty onslaught gave the Titans’ offense life and confidence, translating into points in the second half. And on the other side of the ball, the penalty barrage put Mayfield into a number of difficult situations, prompting him to press and eventually throw three interceptions, the last of which Malcolm Butler returned for a pick-six exclamation point.

“I think everybody knows what the problem is,” Mayfield said. “Dumb penalties hurting ourselves. Then obviously turnovers on my part. Just dumb stuff.”

Yet as dumb as all the penalties were, they overshadowed other troubling manifestations, most notably the play of the offensive line.

Before and after Robinson’s ejection, the Browns struggled to protect Mayfield and give him the time he would need to unlock Beckham and Jarvis Landry downfield. According to Next Gen Stats, all five sacks and three interceptions of Mayfield came when Tennessee brought only four pass-rushers. And of the 27 passing plays in which the Titans rushed only four players, they forced a whopping 10 quarterback pressures.

Offensive line was always going to be the potential Achilles' heel of an otherwise potent Browns attack, a point implicitly reinforced last week when Dorsey traded for a pair of guards in Justin McCray and Wyatt Teller. But once Robinson exited the field, the line began to capitulate, as Mayfield was sacked for a safety on the ensuing drive. From there, it was open season on Mayfield, who after the game had to get to an X-ray on his throwing hand, which revealed a bruise, but no break, according to a source. Still, Mayfield left the stadium with his hand and wrist wrapped.

“We have to evaluate that and see,” said Kitchens, hinting that schematic changes could be on the way to address his line's deficiencies, remedies that will have to come quickly with reigning NFL Defensive Player of the Year Aaron Donald and the Los Angeles Rams -- 2018's NFC champions -- visiting Cleveland in two weeks.

“We have to get the ball out of [Mayfield’s] hand quicker. We held onto the ball for too long. Sometimes [the Titans] were back there pretty quickly, but whatever it is, we have to protect better. I have to do a better job of calling plays where we can protect better and help us protect better.”

The Browns will have to do much better defensively, as well.

Tennessee’s receivers, including rookie A.J. Brown, physically dominated Cleveland’s secondary, especially after the catch. And on a day when the Titans didn’t have suspended left tackle Taylor Lewan, the Browns’ supposed lethal pass rush led by Garrett was noticeably tamed.

“We didn’t even circle this game,” crowed Titans tight end Delanie Walker, who baited Garrett into punching him in the face for yet another backbreaking penalty. “They were who we thought they were.”

Despite Walker’s best efforts, one game isn’t enough to define these Browns.

But after such an inexplicably disheartening defeat, on the heels of an offseason of big expectations and big hype and big talk, a defining moment is already here.

"Everybody's going to throw us in the trash, and I think that's good,” Mayfield said, defiantly. “I know what type of men we have in this locker room, and quite frankly, I don't really give a damn what happens on the outside. I know how we're going to react; I know what we're going to do, how we're going to bounce back.

"We're ready to go."

Who are these Browns and what is their mettle?

We’re about to find out, much sooner than we could’ve imagined.