The team announced the sprain of his MCL after an MRI on Saturday.
Gasol suffered the injury early in the second quarter of Friday night's loss to the San Antonio Spurs. Replays indicated there was little if any contact before Gasol grabbed his knee under the Spurs' basket. He went to the Memphis locker room and did not return.
Gasol was second on the Grizzlies in points, rebounding and assists entering the game.
Kosta Koufos, obtained from the Denver Nuggets in an offseason trade, started the second half in Gasol's place. He's expected to fill in for the 7-foot-1 center.