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

The 49ers aren't sure how serious running back Jordan Mason's shoulder injury is after Thursday night's win over the Seahawks.Head coach Kyle Shanahan said after the game after Mason went to the locker room with the initial injury, he told the staff he'd be fine and was cleared to return to the game. But after one play, Mason said he was in pain and was kept available only if the team needed him in an emergency, which it didn't."He thought he was going to be alright, he went back in and it just hurt so much so he went back out, I think it was the first play. And then he was out just in case of emergency," Shanahan said.Asked if the injury is serious, Shanahan said that will likely be determined by additional testing on Friday."I don't know. You never know. Adrenaline on these days, during games, is a little different. We'll find out tomorrow when that's down and get X-rays."Mason is currently the NFL's leading rusher, with 609 yards this season. He has filled in more than adequately for the injured Christian McCaffrey, but it's possible that next week, the 49ers will be down to third-string running back Isaac Guerendo for their big Week Seven game against the Chiefs.