Halak had missed the past three games due to a groin strain that landed him on injured reserve. Halak had been expected to start Monday's game.
"[Halak's injury] just happened so quick ... it was after the warm-ups," Blues coach Ken Hitchcock said after the 4-1 loss to the Kings. "I think everybody was in a little bit of shock. It is what it is. He had three really good days of practice and then I don't know, somewhere in the warm-up, he pulled it again. It isn't good for us right now ... it's not good for the team right now. We're just going to have to learn to battle through it."
Allen, a 2008 second-round pick, had been called up when Halak previously was injured and served as Brian Elliott's backup. In 31 AHL games this season, Allen is 12-16-2 with a 2.94 goals-against average, .903 save percentage and two shutouts.