Foster experienced some swelling in his left knee and has been downgraded to doubtful for Sunday's game against Tampa Bay, according to coach John Fox.
"He hasn't come along quite as we had hoped and we're just re-evaluating what his status will be," Fox said. "I think he had some discomfort there and a little bit of swelling there. I'll just let the doctors evaluate it and see what the next course of action is. He hasn't come along quite as we had hoped."
That means Lamar Smith will get the bulk of carries on Sunday against the Bucs.
Foster, the 34th player selected in this year's NFL draft, has yet to play in a regular season game after injuring his knee in a preseason game against the New England Patriots.
He had surgery to repair cartilage damage on Aug. 25 and was originally expected to miss only four weeks of action. He's now missed the entire first of the season, and maybe more.
"I think it's frustrating for everybody," Fox said. "I think it's frustrating for DeShaun, number one. It's part of the game and it's something we want to get fixed and fixed correctly. The doctors have done everything in their power to rest it and get it to this point. Maybe it will require something different, I don't quite know at this point."
Foster showed a great burst of speed in the preseason for the Panthers and was the team's most explosive offensive player.
"I think we obviously had a lot of strong feeling on him to draft him in the second round, so he's a guy we were excited about," Fox said. "We wanted to run the ball, so the position is critical and it's unfortunate that he got injured, but it's part of the game and we just have to try to get him well as soon as possible."
The Panthers have a bye following this week's game against Tampa Bay, so Foster will get two more weeks to rest the knee.
"Really, we've got to do what's best for him long term," Fox said. "This type of injury, you can't rush because it will set him back. We had hoped that this rest would heal pretty much on its own. Not that there's been a setback, but it hasn't come along as we had hoped."
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