It started on the opening series of last Thursday's loss to the Patriots.

"I felt myself open up too fast. I tried to correct it, but I could never correct it," Halapoulivaati Vaitai said Sunday.

Vaitai was asked to reflect on the type of performance the third-year Eagles tackle thought he had put far behind him.

"The last time I played that bad was my first game my rookie year," he said. "I'm glad it happened again – another wakeup call. I gotta protect the quarterback."

Vaitai's rookie year of 2016, his first start came in place of right tackle Lane Johnson, on Oct. 16, when Johnson began his 10-game suspension for ingesting a banned substance. Vaitai gave up sacks on each of the first two series. Carson Wentz looked rattled. The Eagles lost.

Vaitai has come a long way since then, the football equivalent of the distance from his parents' native Tonga to Philly, through Texas, where Vaitai was born and raised.

He got five more starts in 2016 and 13 last season, including the three playoff victories. Vaitai was pushed into a critical role by the Oct. 23 season-ending knee injury to left tackle Jason Peters. He was capable, not great, for much of the regular season, but in the playoffs, Vaitai was stellar, a huge reason the Eagles were able to maintain a clean pocket for Nick Foles. Vaitai was having a strong training camp this summer, until Thursday.

"It wasn't 'Big V,' for whatever reason. He's definitely played better," Eagles coach Doug Pederson said Sunday. "We know exactly who he is. He's hard on himself, like anybody is. There are some things he would want back, obviously, but at the same time, he understands what he needs to do to continue to get better and improve."

The Patriots never sacked Foles in Super Bowl LII but they ended his evening and very nearly more than that on Thursday night, when Adrian Clayborn breezed through Vaitai early in the second quarter and hit Foles' arm as he cocked it to pass. The Pats got a fumble recovery for a touchdown and Foles got a game-ending shoulder strain that required an MRI test on Friday. The results were good, and Foles returned to practice when it resumed on Sunday.

"That was a bad read on my part. I should have kept going; I stopped my feet," Vaitai said. "Adrian's a good player. He read it and he just beat me, and I gave up a short edge."

>> READ MORE: Nick Foles' performance on Thursday was nothing special

The trip to New England came at the midway point between the start of training camp and the start of the regular season. It has been a long, hot grind for everyone. Vaitai started Thursday in place of Peters, the oldest starting tackle in the league at 36, who is healthy but "very limited" in camp as a precaution, Pederson said.

"I wasn't ready mentally. I guess it was one of those deals where you just get mentally tired, but I shouldn't use that as an excuse, I should be mentally ready every time," Vaitai said.

"I wasn't doing what [offensive line coach Jeff Stoutland] asked me to do … I just wasn't myself. I couldn't bounce back. Usually, I bounce back. I couldn't, all night."

When you're 6-foot-6, 320 pounds, smaller defensive linemen can gain leverage, especially if you make a mistake or two and let yourself become tentative. Vaitai said that during the game, Peters was telling him: "Hey man, try to sit down. You're playing too high."

Teammates empathized.

"When you find yourself in a firestorm, you just have to walk through," Johnson said. "Sometimes you get in one of those games where it feels like the harder you try, the worse you do. So the biggest thing is, you go to the sideline and readjust, take coaching as best you can. I've been in those games. It sucks. But those are the ones you learn the most from."

Center Jason Kelce recalled how he had to reboot his career after a poor season in 2015. Kelce said o-line play can be "so technical, so detailed, if you lose track of the proper technique, if you lose track of what's allowed you to be successful in the past — we've all been through spells. Obviously, everybody knows, mine's pretty well documented. [Peters] went through a year [also 2015] where even he was struggling, and he's one of the best of all time."

If there was a bright spot, Vaitai said, it was that he got jolted out of his comfort zone in the preseason, rather than in a game that counted. But that didn't make his film session with Stoutland any easier.

"I was just embarrassed," he said. "I didn't even want to watch it, but I had to."