San Francisco 49ers running back Jordan Mason, the NFL’s leading rusher, is believed to have suffered a sprained AC joint that is not considered serious in Thursday night’s win at Seattle, but he will undergo further testing on Friday, sources told ESPN’s Adam Schefter.

Mason injured his left shoulder in the first half of the 36-24 win over the Seahawks, returned for one carry to open the third quarter and was only available on an emergency basis the rest of the game.

He finished with 73 yards on nine carries and caught another pass for 9 yards. On the season, Mason has 609 rushing yards on 114 carries.

The 49ers are already without key players such as running back Christian McCaffrey (Achilles tendonitis), linebacker Dre Greenlaw (torn Achilles tendon), defensive tackle Javon Hargrave (torn triceps) and safety Talanoa Hufanga (torn wrist ligaments).