The forward left a 3-1 loss to the Arizona Coyotes with 10:24 remaining in the first period Saturday after he was tripped by Coyotes forward Nick Schmaltz and fell on his left hand. Thomas scored his first goal of the season at 4:05 to tie the game 1-1.
"I feel awful for him, and we're going to miss him," Blues coach Craig Berube said. "He didn't get off to the start (to the season) that he wanted to, but it was a good start yesterday for him, and it's unfortunate what happened. It's something he's going to have to deal with and something the team's going to have to deal with."
Thomas has scored six points (one goal, five assists) in 12 games this season.
"It's obviously unfortunate," Blues center Brayden Schenn said. "Gets his first goal last night, and you never know what can happen with that gaining confidence and his ability to help our team. Unfortunate for him, unfortunate for us, but he'll obviously be hungry when he gets back, and in saying that, there's other guys that need to step up and fill the void and guys will."
The Blues (7-4-1) host the Coyotes on Monday (8 p.m. ET; FS-MW, FS-A, NHL.TV). St. Louis has lost two straight games for the first time this season.