The Brewers announced the extent of the injury following a 4-3 win over the Marlins.
Yelich was injured in the top of the first inning when he fouled a pitch off his kneecap.
Yelich remained on the ground for several minutes before getting up and limping to the dugout.
The outfielder was last season's National League MVP.
Yelich began the day batting .330 with 44 home runs and 97 RBIs for the playoff-contending Brewers and led the majors in slugging percentage and OPS.