Howard, the 2006 NL MVP, went on the disabled list Saturday. An MRI Monday confirmed the tear in his knee. A date for surgery hasn't been set.
The three-time All-Star was hitting .266 with 11 homers and 43 RBIs. He was noticeably limping during games before he was shut down. He said he continued to play through the pain because he thought he could perform.
Howard played his first game last season on July 6 after tearing his left Achilles tendon making the final out in the 2011 NL division series.