Berube said of Thomas, "Well, he said he could play. He was ready to play, so he feels good enough to play and we need a player.
"He started feeling better and just a little bit of time off has helped too. So when we lost Barbashev, we talked to [Thomas] and he's ready to go."
Thomas will play right wing on a line with center Tyler Bozak and left wing Pat Maroon.
"I'm excited," Maroon said. "[Thomas is] a guy that, he's got fresh legs and he's young, he's 19. He's the guy that drove our line, me and [Bozak]. He's done a really good job of moving his feet, making high-end plays, protecting the puck in the O-zone, making smart plays out of the D-zone.
"I'm happy for him. He's healthy, he's ready to go. You never want to see any of your teammates go down, but I'm happy he gets the opportunity to play tonight."
Thomas said, "I was pretty confident throughout the whole time that I was going to be able to get back in. The longer the series went on, the better chance I had. That's kind of the way it went."
He and the Blues will take the ice with a chance to win the Cup in front of their fans.
"It doesn't get much better than that, especially just talking with some of the guys who have been around a while and this is my first chance at it," Thomas said. "It's definitely a great opportunity for me and I've definitely got to take advantage of it."