The San Francisco 49ers will be without their top two running backs for the remainder of the regular season after both rushers suffered injuries on Sunday Night Football against the Buffalo Bills.

While speaking to members of the media on a conference call on Monday, head coach Kyle Shanahan confirmed that Christian McCaffrey suffered a PCL injury against the Bills. Shanahan said McCaffrey’s injury will not require surgery, but his recovery time will be about six weeks. Shanahan said the 49ers would place McCaffrey on injured reserve.

In addition to McCaffrey, fellow running back Jordan Mason will also be added to the injured reserve. Shanahan revealed that Mason had suffered a severe ankle sprain, forcing him to move back to injured reserve.

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This article originally appeared on Niners Wire: 49ers’ injury update is bad news for 2-star running backs