Allen, 22, will get the call when the Blues face the Red Wings on Wednesday (7:30 p.m. ET, NBCSN, TSN2).
Allen was recalled Tuesday from the American Hockey League's Peoria Rivermen for the second time in a week after Jaroslav Halak aggravated the groin strain that sidelined him for four-plus games during warm-ups prior to Monday's game vs. the Los Angeles Kings.
Allen, the 34th pick in the 2008 NHL Draft, is 12-16-2, with a 2.94 goals-against average and .903 save percentage for the Rivermen this season.
"It's a great feeling," Allen said. "It's something you work for your whole life. You want to make the most of it. I know I'm definitely going to enjoy it. I'll remember it for a while.
"It's another game. I'm excited. ... It's hopefully another step in my career. It's more experience under my belt and I'm looking forward to the challenge tonight."
The Blues are mired in a 0-4-1 slump and will turn to Allen to help them get back on track. With Halak injured, Brian Elliott has lost all five games since replacing Halak ironically here at Joe Louis Arena. He has a 4.67 GAA and .810 save percentage since Halak's injury.
"It's a reset button for Brian," Blues coach Ken Hitchcock said. "We just can't keep going down the same path with Brian because Brian's got to catch his breath here and reset and get back to what he was. We've got to get him calmed down and playing the position because we don't know where Jaro's at."
Hitchcock admitted there's an unknown element entering Wednesday's game by gambling on Allen to help the team out of a slump.
"I don't know. I haven't done that before for a long, long time, so I have no idea how this is going to turn out," Hitchcock said. "This is new ground for all of us.
"He's played really, really well. His last four or five starts in Peoria have been excellent. But this is a different league. We're a lot like Detroit. Detroit brought [Mrazek] in [a 5-1 win at St. Louis on Feb. 7]. He was competitive. I thought the players fed off his competitiveness. That's what we're hoping for Jake. He's got to get his career started somewhere. Other than Toronto, you couldn't get a bigger building to play in. You couldn't get a bigger atmosphere for a young guy to start his career other than probably Toronto."
Allen backed up Elliott last spring in the Stanley Cup Playoffs when Halak got injured against the San Jose Sharks during the first round. He made his NHL debut last week, coming on in relief during a 6-1 loss to the Nashville Predators. Allen gave up two goals on eight shots.