Forward Jaden Schwartz suffered a fractured left ankle Friday at practice that required surgery and he is expected to miss at least 12 weeks.
Schwartz was injured while crashing into the net at practice.
Injuries continue to pile up for the Blues, who were already missing center Paul Stastny (broken foot), forward Robby Fabbri (concussion), defenseman Kevin Shattenkirk (groin) and forward Patrik Berglund (shoulder).
Shattenkirk and Fabbri are expected to return Tuesday against Tampa Bay.
The Blues said Schwartz, who had four assists in the first seven games, had surgery at Washington University Orthopedic Center in St. Louis performed by Dr. Matt Matava and Dr. Jeremy McCormick.
Schwartz will be reevaluated in 12 weeks.
A first-round pick in 2010 selected 14th overall, Schwartz has 62 goals and 139 points in 214 games with the Blues.
The Blues (5-2) play host to the New York Islanders at 7 p.m. Saturday.