Blues general manager Doug Armstrong announced the injury Tuesday. He also said defenseman Jay Bouwmeester has a left ankle fracture and will be re-evaluated in three weeks.
Sanford, 22, joined St. Louis in February after a trade with Washington. The forward had two goals and three assists in 13 regular-season appearances for the Blues, and he also played in four playoff games.
Bouwmeester, 33, is entering his sixth season for St. Louis and had 14 assists and a goal last season.
The Blues open their season at Pittsburgh on Oct. 4.