The Blues captain, who suffered an upper-body injury in the late stages of the second period of a 3-2 overtime loss against the Ottawa Senators on Monday, was not in the lineup against the San Jose Sharks on Tuesday. He is listed as day-to-day.
Forwards Vladimir Sobotka and Jaden Schwartz each missed a second straight game with an upper-body injury. Neither player faced the Senators. The Blues went with seven defenseman Tuesday, with Carlo Colaiacovo at left wing against the Sharks.
"We're going to be in a position where every line has to change," Blues coach Ken Hitchcock said prior to facing the Sharks. "I don't know that you look at it from a depth standpoint. I think recognizing when it's time to adjust your team game and right now, we have to adjust our team game.
"... Getting points when you're not healthy is all about being able to adapt."
Backes played in 274 consecutive regular-season games and 289 including the Stanley Cup Playoffs dating to Jan. 25, 2010.
Backes, 29, collided with Senators forward Colin Greening in the waning seconds of the second period, in what appeared to be a head-to-head collision that sent a wobbly Backes toward the Blues bench. He had trouble getting down the tunnel leading to the locker room and needed help from the training staff.
Backes is second on the Blues in goals (16) and points (30) behind team leader Alexander Steen (22, 35).