The injury does not require surgery, the source added, though Reagor is seeking a second opinion.
He was hurt during Sunday's scrimmage at Lincoln Financial Field, which marked the end of training camp. Reagor's left arm was extended backward as he tried to tackle safety Will Parks following an interception. He was taken inside after being examined by athletic trainers on the sideline and then underwent an MRI, which revealed the small tear.
Reagor, the Eagles' first-round pick in April's NFL draft, has been a bright spot at camp, making plays as a receiver on the first-team offense while cementing his role as the team's punt returner. It appeared coach Doug Pederson was going to give him a big role right out of the chute.
With Reagor sidelined, second-year player J.J. Arcega-Whiteside likely will get the bulk of the reps on the outside opposite DeSean Jackson, with Greg Ward manning the slot. Fellow rookie John Hightower has been a camp standout as well and could find a role in the offense.
Alshon Jeffery is currently on the physically unable to perform list as he rehabs from Lisfranc surgery, but Pederson has expressed optimism when it comes to the veteran receiver, saying recently that he has picked up his running of late and that the Eagles are "hoping to get him out there soon with the team."