Allen, who is 26-15-3 with a 2.35 goals-against average and .930 save percentage in 47 games (six shutouts), sustained an undisclosed injury in the first period of the 5-1 win against the Colorado Avalanche on Sunday. He was injured on a play late in the period and played the final three-plus minutes but did not come back and was replaced by Anders Nilsson.
Backes is second on the Blues with 21 goals and has 45 points in 79 games. Backes and forward Troy Brouwer are the only Blues players to skate in every game.
"They're hurt. They're not able to play," coach Ken Hitchcock said. "We'll give you an update on the time frame on both guys on Monday, but not able to play, both were hurt [Sunday]. Can't tell you time frame-wise because we don't honestly know. Obviously a sense of urgency with playoffs around the corner is there, but believe me, if they were able to play, they'd be playing. Not able to play."
The Blues play the Arizona Coyotes on Monday (8 p.m. ET; FS-A PLUS, FS-MW, NHL.TV).
Nilsson will be the backup for Brian Elliott for the final three regular-season games.
The Blues have clinched a berth in the Stanley Cup Playoffs and are two points behind the Dallas Stars for first place in the Central Division and the Western Conference.
Hitchcock said they'll have a better idea by Monday if they'll be available for the start of the playoffs.
"You know what the playoffs are like. You try and play," Hitchcock said. "If you've got to play at 80 percent, you play at 80 percent. You're hopeful that you're at 100 (percent), but none of us can honestly tell you right now until these things are moved a little bit further down the dial here. It's not what we hoped for, it just seems to be what our season's been about and we've got to step up."
