JaCorey Shepherd was carted off the field during a training camp workout Sunday, and the rookie cornerback is due for an MRI exam on his right knee Monday to determine the severity of the injury.

If the diagnosis proves to be a torn knee ligament, it would be a tough loss for the Eagles' secondary. Shepherd, a 22-year-old, sixth-round draft pick, is the front-runner to replace Brandon Boykin as the team's top nickel cornerback. He has had a strong training camp and stood out to the coaching staff since the spring.

The injury occurred along the sideline at Lincoln Financial Field during 7-on-7 drills. Darren Sproles caught a pass and Shepherd put a "thud" on him - the Eagles do not tackle during camp. The two collided, with Sproles staying upright and Shepherd falling to the ground.

"I kind of braced myself, and I heard him hollering," Sproles said after practice. ". . . You don't want [anybody] to go down. We're all a team. We need everybody now."

Sproles said he didn't know what caused the injury. He wondered whether Shepherd's foot was caught in the turf when they collided. Shepherd remained on the ground for a few minutes and practice was halted for a moment. He could not stand when he tried and a cart was needed to bring him to the locker room.

Jaylen Watkins took the snaps as the first-team nickel cornerback Sunday. He filled that role throughout the day - it was his turn in the rotation, which lasts three days. Shepherd started camp in that spot. E.J. Biggers is also in the rotation.

If Shepherd's injury keeps him out for an extended period, Watkins would be a likely candidate to replace Boykin. He was a fourth-round pick in 2014 who did not contribute much as a rookie. Watkins, 23, is viewed as a cornerback/safety by the coaches. At 5-foot-11 and 194 pounds, Watkins has good size for the nickel spot.

"I'm a bigger slot corner," Watkins said. "I can play the run game better. I play safety, so obviously I fit the run game. I'm one of the better cover guys in general in the slot. . . . The slot is a lot of [the] mental aspect of the game."

Biggers, 28, has starting experience at cornerback and safety with Washington and Tampa Bay, and also has played the slot. Other possibilities could be sixth-round pick Randall Evans and undrafted rookie Denzel Rice, who has also caught the attention of the coaches this summer.

When the Eagles traded Boykin, they were confident in the other cornerbacks on the roster. Shepherd, a former Kansas star, appeared to be a big reason for that optimism.

"We are really, really impressed with him," coach Chip Kelly said last week. "I think he slipped a little bit in the draft because he was hurt. . . . But when you turn the tape on, he's a legitimate football player. What we saw on tape is what we saw during OTAs and minicamp and the offseason program."

The Eagles already lost linebacker Travis Long to a torn anterior cruciate ligament during the first week of training camp. Shepherd's MRI could reveal whether he is another intriguing option who is sidelined before the preseason even begins.