After three Eagles tore knee ligaments on the same practice field at the NovaCare Complex in a two-week span, coach Chip Kelly is trying to determine if there's an explanation.

All three fields at the team's training facility were recently resodded, but all three injuries occurred on one field.

"We're looking into everything," Kelly said. "Obviously, when you have the same injury and it happens in a two-week span . . . we're going to look at it and try to figure out, obviously, is it preventable? Is it something that we can make sure doesn't happen?  But I don't have any information. I know we're looking at it."

Injuries are not uncommon this time of year, and a daily injury report around the league reveals players being lost for the season. But the fact that Jeremy Maclin, Jason Phillips, and Arrelious Benn all suffered torn anterior cruciate ligaments, and contact wasn't involved in any of the injuries, makes this a particularly curious case.

Injury report

Five players left Wednesday's practice early: running backs LeSean McCoy (knee) and Felix Jones (hamstring); wide receivers Damaris Johnson (hamstring) and Russell Shepard (hamstring); and cornerback Cary Williams (hamstring).

Offensive lineman Nate Menkin missed practice with an illness. Defensive tackle Antonio Dixon (hamstring), and offensive tackles Jason Peters (hamstring), Dennis Kelly (back), and Ed Wang (knee) all added another day to continued absences from practice.

McCoy's injury is not believed to be serious, and injuries to him and Jones gave Bryce Brown and Chris Polk the bulk of the carries. The injuries to Johnson and Shepard left the Eagles with seven healthy wide receivers by the end of practice.

Williams' injury continued his pattern of missing time, which dates back to the spring. He missed part of the offseason program for personal reasons, sat with a hamstring injury earlier in camp, and left Tuesday's practice early after getting into a fight with a Patriots receiver.

Benn officially out

The Eagles officially placed Benn on injured reserve on Wednesday, one day after his season-ending injury. The Eagles did not make a corresponding move. They also did not make a move to replace Dave Ball, who was released Tuesday. And they still had one leftover roster spot from last week, leaving the team with three unused spots on the 90-man roster.

Kelly acknowledged that the Eagles are looking for one or two receivers to help fill depth, but they do not appear to be in a rush. The result was practicing with an undermanned unit.

"But on game day, we're only going to have five, probably," Kelly said. "So we've got the luxury of two more than we would have if we played on Sunday."

Three-safety set

The Eagles have been experimenting with a three-safety formation that slides Patrick Chung over to the slot. He played it on Wednesday, bouncing normal slot cornerback Brandon Boykin to the outside.

The Eagles experiment with personnel groupings, but Chung is expected to be a starting safety, so the move is noteworthy.

Kelly praised Chung's intelligence and ability to train at different positions.

"If [Boykin] goes down, then what are you going to do?" Kelly said.  " . . . So we have to develop at that position."

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