Thomas suffered the injury during Game 6 of the Western Conference Final against the San Jose Sharks. He will rehabilitate the injury throughout the offseason and will undergo a full evaluation ahead of training camp.
Thomas, 19, appeared in 70 regular-season games in 2018-19, posting 33 points (nine goals, 24 assists) and 14 penalty minutes. The 6-foot, 188-pound forward also collected six points (one goal, five assists) in 21 postseason games, helping the Blues capture the Stanley Cup in 2019.