49ers' Jordan Mason Diagnosed with AC Joint Sprain; Day to Day with Shoulder Injury

The San Francisco 49ers have avoided the worst-case scenario with running back Jordan Mason.

49ers head coach Kyle Shanahan told reporters Friday that Mason was diagnosed with an AC joint sprain and is day to day with the injury.

Mason left the game in the second quarter, of the 49ers 36-24 victory over the Seattle Seahawks on Thursday. although he did return to the sideline in the second half. He finished the day with nine carries for 73 yards.

While Mason's initial diagnosis appears to be positive, missing any time would be a major blow to the team's tailback depth. Mason already was filling in for Christian McCaffrey, who has not played this season as he battles Achilles tendinitis.

McCaffrey is not expected back for at least another month, according to NFL Network, and losing Mason would force Isaac Guerendo into the starting job. Guerendo had 10 carries for 99 yards in the Week 6 victory over the Seahawks but had 76 yards on one carry and is a rookie.

Mason has been excellent during McCaffrey's absence, dashing for 609 yards and three touchdowns across the first three games. San Francisco has opened the season 3-3 and is hoping to string several wins together and reclaim a hold on the AFC West.

The 49ers have a Super Bowl rematch against the Kansas City Chiefs in Week 7 and will hope the extra rest will help Mason return to the lineup for that game.