Jeremy Maclin, his face covered by a towel, was carted off the field at the NovaCare Complex after he suffered an injury to his right knee during practice on Saturday.
The wide receiver was being evaluated by the Eagles' medical staff and undergoing an MRI, the team said.
Maclin was on the ground for about five minutes as Eagles trainers tended to him. Players were visibly upset and coach Chip Kelly stood nearby as Maclin lay flat on his back.
The injury occurred during 7 on 7 drills. Maclin took a few steps off the line of scrimmage and dropped to the ground, according to several eyewitnesses. Michael Vick was the quarterback on the play.
"Usually when a guy falls like that there's something to be worried about," Vick said.
The players aren't yet wearing pads so there hasn't been contact yet at training camp.
Maclin was able to get up and stand on his left leg, but had to be helped into the back of a waiting cart. As he was being carted off, tackle Jason Peters, who missed last season after he ruptured his Achilles tendon, patted Maclin on his shoulder.
Maclin is entering the final year of the five-year contract he signed as the Eagles' top pick in 2009. He said on Thursday that he was looking forward to proving what he is worth this season.
Riley Cooper moved up and took the injured Maclin's snaps with the first team.