San Francisco 49ers RB Jordan Mason, the NFL’s leading rusher, is believed to have suffered a sprained AC joint during Thursday night’s win at Seattle that is not considered serious, but he will undergo further testing 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 then was only available in an emergency situation 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 tendinitis), linebacker Dre Greenlaw (torn Achilles), defensive tackle Javon Hargrave (torn triceps) and safety Talanoa Hufanga (torn ligaments in his wrist).