The linebacker said he believes he sustained the injury when he hit his foot into the turf while chasing a play.
Willis was replaced at middle linebacker by rookie Chris Borland. The 49ers were already playing without starting linebackers NaVorro Bowman, who tore his ACL and MCL in last season's NFC Championship Game against the Seattle Seahawks, and Aldon Smith, currently serving a nine-game suspension due to multiple off-field issues.
An eighth-year pro, Willis has made the Pro Bowl every season in his career and has led the NFL in tackles in two seasons. He leads the league in tackles over the past six seasons and has 34 tackles and one interception in six games this season.
It's currently unknown how much time Willis might miss due to his injury.
The 49ers are 4-2 this season and in second place in the NFC West.
- Ben Estes