The Blues' 2-1 overtime victory on Saturday night was further evidence to coach Ken Hitchcock that the 22-year-old Allen is for real.
"He knows he's good enough to play and he's not going to give up anything easy," Hitchcock said. "If he keeps playing like this, we've got to keep playing him.
"We're in playoff mode right now and we need all the points we can get."
First, the Blues had to survive a third period in which they were outshot 9-2.
"I know I had some big saves there, but that's my job," Allen said. "It's still a little bit of a whirlwind, just enjoy it tonight."
Stewart, who assisted on the Blues' other goal by Kris Russell, has seven goals and seven assists in his last seven games, including two-goal outings the previous two games. He took the lone shot of the extra period, faking Jonas Hiller with a forehand in front and then finding plenty of space with a backhander for his 14th goal of the season.
Sobotka's patience set up the play. After skating into the offensive zone, he skated around sliding defenseman Cam Fowler and waited until Selanne also slid helplessly past, too, before feeding to Stewart in front.
"By the time he gave it to me, Hiller was already down and out," Stewart said. "I went back to the backhand and put it home.
"Maybe I over-celebrated, but I was really excited. This was a great feeling, a big two points."
Saku Koivu scored on a first-period deflection for the Pacific Division-leading Ducks, who are 13-2-2 in their last 17 games to tie for the best run in franchise history. Anaheim outshot St. Louis 28-20 in regulation.
"We've played every second day since Feb. 24 and we still have five games next week," Ducks coach Bruce Boudreau said. "For the team just to play as good as they've been playing consistently is pretty amazing to me."
The Blues went 3-0 on their homestand, turning around early season struggles when they won just four of their first 10 in St. Louis.
Allen has won eight of his nine career starts with a 2.18 goals-against average and .920 save percentage since getting a chance to play due to Elliott's slump and inconsistency from Halak.
Hiller had surrendered just two goals on 65 shots the previous two games. The Ducks had outscored opponents 14-4 during the five-game winning streak, including a 4-2 victory over St. Louis at home.
The Ducks applied heavy pressure the last several minutes of the first period and it paid off with 20 seconds to go when Bryan Allen's shot from the left point ticked off Koivu in front. Koivu ended a six-game goal drought and earned his first point in four games. Allen, initially awarded his first goal of the season, got his third assist.
The Blues tied it on Russell's first goal since April 7, 2012, at 12:42 of the second. Russell jumped into the play off the bench and caught the puck in stride off a pass from rookie Vladimir Tarasenko, who returned to the lineup after missing 10 games with a concussion.
Boudreau complimented both goalies, saying "Anything they could see, they stopped. Hiller didn't see Russell's goal for sure. Our goal was tipped by Koivu, I'm sure Allen didn't see it.
"Those are when two goalies are zoned in."
The Ducks have outscored the opposition 33-17 in the second period. ... Corey Perry, second in scoring for the Ducks with 24 points, served the second game of a four-game suspension. ... Standing room attendance of 19,153 was the Blues' fourth sellout. ... Anaheim's Bobby Ryan appeared to get the worst of a nasty-looking mid-ice collision with Sobotka in the third. "It shook me up and took the wind out of me and it took a while to get it back," Ryan said.