Kyle Shanahan says it's unclear how serious Jordan Mason's shoulder injury is

The 49ers aren't sure how serious Jordan Mason's shoulder injury is after Thursday night's win over the Seahawks.

Head coach Kyle Shanahan said after Mason went to the locker room with the initial injury after the game, he told the staff he was fine and was cleared to return to the game. But after one game, Mason said he was in pain and was only being made available if the team needed him urgently, which was not the case.

“He thought he was going to be OK, he went back and it hurt so much that he went back, I think that was the first play. And then he was out in an emergency,” Shanahan said.

Asked if the injury was serious, Shanahan said that would be determined by additional tests on Friday.

“I don't know. You never know. Adrenaline these days, during games, is a little different. We'll find out tomorrow when it's closed and do an x-ray.”

Mason is currently the NFL's leading rusher, with 609 yards this season. He's filled in a lot for the injured Christian McCaffrey, but it's possible that next week, the 49ers could drop Isaac Guerredo to the third string for their big Week Seven game against the Chiefs.