"It doesn't go numb," Foster said on Thursday via Cam Inman of Bay Area News Group. "It's a sharp pain that goes on and off. I want to stay out there the whole time. I listen to my coaches and trainer. Whatever they decide for me, I know it's best."
Foster's play has suffered. He has missed 12 tackles this season, which ranks as the second-most among NFL defenders. Foster has also been a liability in pass coverage. Opposing quarterbacks are 20-of-27 for 228 yards with two touchdowns and a passer rating of 123.7 when targeting the linebacker.
A recent MRI revealed no structural damage. Foster doesn't anticipate needing surgery during or after the season.
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"It was just tweaked, probably fluid, and something you have to continue to rehab," Foster said.
General manager John Lynch, who joined KNBR on Friday morning, doesn't believe the injury to be a long-term issue. He was asked if shoulder issues could plague Foster his whole career.
"No, we don't feel like that at all," Lynch said on the "Murph & Mac" show. "When we drafted him, he had had rotator cuff surgery, and they repaired that. That repair has been good. It's been intact.
"Against Arizona, I think Reuben had a scare. He was making a tackle, and he strained his shoulder.
"I can promise you, we never put anyone out there who's not cleared. With that, there's a big process where you have to prove that you have your strength, that you bring down the swelling, all those things. Reuben's been checked out. A lot of people saw him running around the Arizona game, not being able to use that shoulder. I think it has affected his play.
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"I think also what has affected Reuben's play is he had a tumultuous offseason with things going on, and he didn't get to have that improvement opportunity that players need after that rookie year, that everybody needs. I think we're seeing, unfortunately, some of that.
"Reuben needs to be an impactful player for us. That year, in that draft, I thought he was one of the better players in that draft. We were fortunate to get him where we got him, and those are things that need to take place. You have to draft really well, and then you have to develop your players.
"Reuben will start playing better football. I'm very confident in that. He takes his job seriously. He's healthy. He's playing through something with his shoulder because it is strained, but he's able to be out there, and I think we'll start to see Reuben Foster be a more impactful player. I'm very confident in that."
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