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Mason is believed to have suffered a sprained AC joint in his left shoulder during Thursday's 36-24 win over the Seahawks but will undergo further testing Friday, Adam Schefter of ESPN reports.

Mason exited Thursday's game in the second quarter, returned for one play after halftime, and then missed the rest of the game. Isaac Guerendo filled in as San Francisco's lead back and iced the game with a 76-yard run in the closing minutes, with Patrick Taylor also taking on some work (five carries, one catch) in Mason's absence. Meanwhile, the 49ers hope to get Christian McCaffrey (Achilles) back at some point within the next few games, although it's not clear if he has any chance to be ready for a Week 7 matchup with the Chiefs on Oct. 20.

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