Report: 49ers' Mason believed to have sprained AC joint

San Francisco 49ers running back Jordan Mason is believed to have sustained a sprained AC joint in the team's 36-24 win over the Seattle Seahawks on Thursday, sources told ESPN's Adam Schefter.

Mason will undergo further testing on Friday, though the injury isn't considered serious, Schefter adds.

The 25-year-old missed the majority of the second half against Seattle.

Mason has been a key contributor to the 49ers' offense with reigning Offensive Player of the Year Christian McCaffrey sidelined to begin the campaign. The Georgia Tech product leads the league in both rushing yards (609) and carries (114) and also has three rushing touchdowns.

Rookie Isaac Guerendo and Patrick Taylor are next in line for running back snaps if Mason is forced to miss time and McCaffrey remains sidelined. The 49ers also have veteran Ke'Shawn Vaughn on their practice squad.