McCormick scored his first goal of the season in a 2-1 loss at Tampa Bay Lightning on Friday, but is now set for an extended spell away from the ice.
Coach Ted Nolan told the Buffalo News: "McCormick had a situation where he felt something, the medical staff looked at him and he went to the hospital yesterday.
"He has a blood clot in his calf muscle, so he's at the hospital now.
"We're just kind of glad we caught it, and he’s getting medical care right now.
"I'm quite sure it's somewhat serious if he's still in the hospital."
Torrey Mitchell is expected to take McCormick's place after missing 12 games with a lower-body injury.
The Sabres have a few days off before hosting the Detroit Red Wings on Tuesday.