Kelly addressed the media Tuesday:
"It's a big factor," Kelly said of Polk missing time, according to NJ.com. "You gotta be able to play. It's tough for us to say, we know what you can do, but you also have to go out on the field and show it.'
Polk, 24, rushed for 98 yards and three touchdowns on 11 carries in 15 games with the Eagles last season. He went undrafted out of Washington in 2012.
Polk, who has been running away from regular team drills at practice, according to reports, said he fears re-aggravating the injury if he returns prematurely.
The Eagles' next preseason game is Thursday against the Pittsburgh Steelers.