Blues coach Andy Murray said Monday that McDonald was hurt when he crashed into the corner boards late in the first period.
MRI results revealed no further damage and surgery is not required. He isn't expected back until at least mid-January for a Blues team that is currently 15th in the Western Conference standings.
"I was kind of right there. I just heard him screaming and you could tell," said winger Brad Boyes, who has played regularly on the top line with McDonald. "He was in agonizing pain. You could tell right away something was wrong."
McDonald, who leads the Blues with 18 points, had scored five goals in five games prior to Sunday. He appeared to be avoiding taking a stick to the face when he suddenly buckled awkwardly into the corner and went down immediately. He had to be helped off the ice and did not put any weight on his left leg.
"As soon as you see the way he's going down, you're saying, 'Oh, wow,'" Murray said. "I'm taking the glass half full, and that's what I expect everybody to believe and I told the trainers that and our players. That's exactly the way we're going to go."
The injury is the latest in a string of unfortunate and strange injuries the Blues have suffered this season.
They lost cornerstone defenseman Erik Johnson for the season when he tore ligaments in his knee on a golf outing, goalie Manny Legace injured his hip and groin on Oct. 24 while stepping onto a carpet on the ice during pregame warm-ups the night Alaska governor Sarah Palin was to drop the ceremonial first puck, and rookie center T.J. Oshie suffered a high ankle sprain on Nov. 1 immediately after taking a faceoff in a game against Pittsburgh.
"It's just unfortunate weird stuff happens for us to get hurt," said Legace, who has since returned from his injury. "Guys falling out of golf carts, a guy slides into the boards, some donkey trips over a carpet, stupid stuff like that. It's unfortunate and it's a part of the game, but guys have got to step up now."