Examine all the grassy knoll footage you want. Spin every alternative theory you can muster. But Nick Foles practiced Monday, and barring something catastrophic, he is going to be the Eagles' backup quarterback when the regular season opens Sept. 10 at Washington.

According to a source with knowledge of the situation, early in training camp Foles was found to have a flexor tendon strain and inflammation around a nerve in his throwing elbow. The ulnar collateral ligament, which is the part of the elbow that merits Tommy John surgery when it has to be fixed, was fine.

Foles grappled with this same problem last year in Kansas City, the source said, so the Eagles' medical staff gave him a lot of rest during training camp and the preseason, hoping the soreness would go away. Apparently, when Foles did a lot of throwing early last week in the joint practices with Miami, he aggravated the problem, but not to the extent that anyone got concerned.

Before practice Monday, offensive coordinator Frank Reich continued Doug Pederson's message from Sunday, expressing little worry over Foles not having played in a preseason game and showing strong faith in Foles as Carson Wentz's No. 2. Pederson told reporters Sunday that Foles' preseason inactivity would not affect whether the team keeps two quarterbacks or three in the cutdown to the regular-season roster limit of 53, which must be completed by 4 p.m. Sunday.

"Nick's worked good this offseason … I think all players kind of have bumps and bruises along the way," Reich said. "He's had a little bump. I think it'll pass quickly, and we're ready for the season."

Reich indicated that the coaches don't need to see anything from Foles in the preseason to have confidence in him.

"He's proven so much in his career. He's been a winner at this level, proven that he can win big games," Reich said. "So we're confident."

Last year the Eagles kept Carson Wentz and backup Chase Daniel on the 53 and then stashed another QB, Aaron Murray, on the practice squad, to be activated if there was an injury. This year's third QB, former Penn State star Matt McGloin, is approaching his fifth NFL season and does not retain practice squad eligibility.

There would be a $7 million dead cap charge for releasing Foles, on top of the $6.1 million dead cap charge incurred when the Eagles released Daniel.

Back on the corner

When training camp began, one of the storylines was that Jaylen Watkins seemed to have extended his Eagles career by switching last year to safety from corner, which he was drafted to play, in 2014's fourth round.

Welp, the Eagles have more depth at safety this year than last, as you might have noticed over the weekend when they dispatched Terrence Brooks to the Jets for corner Dexter McDougle. Corey Graham is definitely the first guy up behind starting safeties Malcolm Jenkins and Rodney McLeod, a role Watkins performed last season.

But the Eagles still seem unsure exactly what they have at corner, after opening training camp without a proven NFL starter, then trading for Ronald Darby and McDougle. So, lately, Watkins has been back at corner. And he's been doing well.

"One of the signs that you're playing well at corner is that nobody's noticing you," defensive coordinator Jim Schwartz said Monday. "I think, other than the interception thatJaylen had, he hasn't been noticed. That means he's doing his job out there.

"It's a little bit like, an offensive lineman gets noticed when he gives up a sack or something like that. Corners get noticed when they either make a big play or give up a big play. Jaylen hasn't given up plays, and he also has made a couple for us."

Schwartz said he especially liked Watkins' end zone interception of Dolphins backup QB Matt Moore in the second quarter of last week's game.

"We didn't play the route very well inside, but Jaylen did a good job recognizing it, being where he was supposed to be, and not only getting the interception, but flipping field position a little bit there [with a 50-yard runback]."

Birdseed

Jim Schwartz said new corner Dexter McDougle is enough of a veteran that his biggest learning challenge this week will be terminology … Frank Reich said he is pleased with how Halapoulivaati Vaitai is performing in his role as the swing tackle, the first tackle off the bench on either side. "I think V has made a lot of progress. He's playing more aggressive. When he came out of college [at TCU], the system he played in, it was kind of a pass-only, two-point-stance only." Asked about the three penalties in four snaps Vaitai was flagged for against Miami, Reich said one of them wasn't really on Vaitai, and that overall, they were "a real small blip on the radar screen." … Among those sitting out practice were Brandon Graham (triceps strain),  Jordan Hicks (groin strain) and Najee Goode (hand).