The 33-year-old Steen was hurt against Dallas on Tuesday night and will be re-evaluated in three weeks. He is entering his 10th season with the Blues and coming off a year in which he had 16 goals and 35 assists in 76 regular-season games.
The Blues are already without forward Zach Sanford, who will miss five to six months after dislocating his left shoulder during training camp. Defenseman Jay Bouwmeester also has a left ankle fracture and will be re-evaluated in three weeks.
The Blues open their season at Pittsburgh on Oct. 4.