He may be out of the lineup when the best-of-seven Western Conference quarterfinal series between the two teams resumes Monday in St. Louis.
A Blues' spokesman described the injury as an "upper body injury," but the team recalled defenseman Tom Koivisto from Worcester of the American Hockey League.
Koivista, 28, a native of Finland, had two goals and four assists in 22 games for Worcester.
MacInnis was hammered into the boards by 245-pound Vancouver left wing Todd Bertuzzi early in the first period. He immediately favored his right arm, went to the locker room, and did not return.
Vancouver evened the series at one win apiece.
MacInnis took over as team captain this season, when former Hart and Norris Trophy Chris Pronger missed all but five games following wrist surgery. He held together a defense that played in front of seven goaltenders, and helped Barret Jackman round into a Calder Trophy candidate.
"He's a big part of our team," Jackman said. "Anytime you play a shift, a period, or three periods without him, everyone's gotta step up."
MacInnis led all NHL defensemen in scoring during the season with 16 goals and 52 assists, and likely will be a finalist for his second Norris Trophy.
"We've had a lot of injuries over the course of the year and we seem to find a way to win," said Pronger. "We're going to have to do it, but it's obviously a big loss for us. He's an integral part of our team and our leader, so we're going to have to find a way to fill in."