Coach Kyle Shanahan confirmed Friday that Mason suffered an AC joint sprain in his left shoulder during Thursday's 36-24 win at Seattle and is considered day-to-day, Eric Branch of the San Francisco Chronicle reports.

EDGE Analysis

Mason exited in the second quarter Thursday after landing on his left shoulder at the end of a carry and clearly favoring it as he went to the 49ers' sideline. While he was deemed questionable to return, he logged one play after halftime before departing the game for good. The 49ers never gave Mason a designation after that, and he was seen on the sideline wearing his helmet the rest of the second half, but Isaac Guerendo handled most of the RB reps for San Francisco down the stretch. The 49ers don't play again until Sunday, Oct. 20 against the Chiefs, so Mason will have extended time to try to get healthy ahead of that contest.