The club gave no indication of how long he would be sidelined.
In the second inning against the Cleveland Indians, Varitek rounded the base after taking second on Coco Crisp's single. He stepped gingerly on his way back to second and then scored on Wily Mo Pena's triple.
Varitek stayed in the game in the top of the third, but Doug Mirabelli pinch-hit for him in the bottom of the inning and went in to catch in the fourth.
Boston's captain was appearing in his 1,000th game at catcher, the 106th major-leaguer to do that. Carlton Fisk, who played with Boston and the Chicago White Sox, is the career leader with 2,226.
Varitek had played in 85 of Boston's 104 games and is hitting .243 with 11 homers and 50 RBI. In 30 games before Monday, Mirabelli was hitting .182 with three homers and 14 RBI in 77 at-bats.
Varitek had been struck on the knee in the last inning of Saturday's 7-6 win over the Los Angeles Angels in 11 innings when he blocked home plate and tagged out Mike Napoli on a throw from left fielder Manny Ramirez.
Information from The Associated Press was used in this report.