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49ers inside linebacker Patrick Willis was ruled out of Monday night’s game at St. Louis with a toe injury.

Rookie Chris Borland replaced Willis in the third quarter.

The nature and extent of Willis’ injury are not known. The 49ers were already without their other star inside linebacker, NaVorro Bowman, who is recovering from a knee injury suffered in the 2014 NFC Championship game.

A seven-time Pro Bowler, Willis has missed just six games in his first eight NFL seasons. He entered Monday night with 31 tackles in five games.

UPDATE 10:39 p.m. ET: The 49ers have announced Willis is out. ESPN had first announced Willis wasn’t returning for the second half.

UPDATE 11:15 p.m. ET: The injury appears to be to Willis’ left foot. According to the Sacramento Bee, Willis is sporting a walking boot on the foot.