Serving as the designated hitter in Monday's game against the Toronto Blue Jays, Nunez appeared to aggravate a previous right knee injury during a swing in the third inning.
Nunez's knee gave out after hitting a foul ball. He fell to the ground in pain, but managed to talk his way into remaining in the game to finish in the at-bat. After lining out to third base, Nunez hobbled off and was spotted walking back to the clubhouse. Sam Travis came on to pinch-hit for Nunez in the fifth inning.
Nunez suffered a sprained right knee on a slide on Sept. 9 that had sidelined him for two weeks.
The 30-year-old entered Monday's game hitting .319/.351/.534 with eight home runs and 11 doubles in 37 games with Boston.