Willis landed awkwardly after missing a tackle on fullback Brit Miller, who made an 18-yard completion from A.J. Feeley with 4:28 left in the first quarter. Willis' body twisted as he fell to the ground at Candlestick Park and he stayed down for a short time as trainers ran out to check on him, then helped him limp off the field.
Willis watched the second half on the sideline in a sweatsuit, but coach Jim
Harbaugh said after the game that he didn't consider Willis' injury to be serious.
Willis came into Sunday's game with 93 tackles, two sacks, a career-best four forced fumbles and an interception for the NFC West-leading 49ers.
Even without Willis, the 49ers won 26-0 and clinched their first NFC West crown and postseason berth since 2002.
Information from The Associated Press contributed to this report.