49ers RB Jordan Mason (shoulder) to undergo tests

San Francisco 49ers running back Jordan Mason is scheduled for tests to determine the severity of a shoulder injury that knocked him out of Thursday's win at Seattle.

Head coach Kyle Shanahan said Mason, starring as the fill-in for injured starter Christian McCaffrey with an NFL-leading 114 carries and 609 rushing yards, had X-rays on Thursday after being hurt on his eighth carry. He tried to return to the game but Shanahan said Mason felt significant pain.

ESPN reported on Friday that Mason is believed to have a sprained AC joint.

"He thought he was going to be all right," Shanahan said of Mason, who finished with 73 yards on nine rushes. "He went back in and it just hurt him too much, so he went out.

"I mean, adrenaline during games is a little bit different. We'll find out more (Friday) when (tests are) done and get the X-rays and stuff."

McCaffrey hasn't played this season because of an Achilles injury and his return date hasn't been discussed publicly by the 49ers. Isaac Guerendo stepped into the lead back role with Mason out.

Guerendo had 99 yards on 10 carries in the 36-24 win over the Seahawks, including a game-sealing 76-yard run late in the fourth quarter. Guerendo has a track and wide receiver background and played collegiately at Wisconsin and Louisville.