Coach Ted Nolan provided the update following practice Sunday. Nolan says McCormick was fortunate the clot was discovered when it was, and adds it's uncertain how long he will miss.
McCormick scored his first goal of the season in a 2-1 loss at Tampa Bay on Friday.
Nolan says center Torrey Mitchell is set to return after missing 12 games with a lower body injury.
Captain Brian Gionta returned to practice, but is not yet ready to play. Gionta has missed nine games with an upper body injury.
The Sabres are off until Tuesday, when they host the Detroit Red Wings.