The chances of Sam Bradford playing Sunday against the Tampa Bay Bucs diminished significantly Thursday with the news from coach Chip Kelly that his starting quarterback still hasn't cleared the concussion protocol.

Bradford suffered a concussion in the third quarter of Sunday's 20-19 loss to Miami. He also suffered a Grade II AC sprain of his left shoulder.

Running back Ryan Mathews, who also suffered a concussion Sunday, hasn't been cleared to return, either.

As for the ongoing saga of left tackle Jason Peters, who has missed the last two games because of a back injury, Kelly said before practice that he wasn't going to train Thursday.

"He's still going through rehab for his back,'' Kelly said.

But 15 minutes later, lo and behold, Peters was on the practice field with his helmet and his ankles taped.

Asked how critical not being able to practice Thursday was for Bradford as far as the installation for Sunday's game, Kelly said, "It's important to everybody. Our work days during the week are Tuesday, Wednesday, Thursday. It's an important day for everybody.''

Kelly acknowledged that if a player at any position doesn't practice on Wednesday and Thursday, it "sets you back a little bit because you didn't get a chance to do things full-speed in the game play.'' But he said there is no set rule that if you don't practice on those days you can't play on Sunday.

Kelly said Bradford still hasn't progressed far enough in the concussion protocol to do any weight lifting or cardio work.

"He's just in meetings,'' he said.

Asked how Bradford's injured left shoulder was doing, Kelly said, "I have not had (an update). The concussion is the No. 1 thing right now. He can't do anything until he's cleared for his concussion.''

Bradford again watched practice, held indoors because of rain. Connor Barwin (quad) was practicing Thursday, for the first time this week. Wideout Josh Huff was not practicing.