The team disclosed the injury Tuesday. The 33-year-old McDonald was hurt during the Blues' 2-1 overtime loss Saturday in Edmonton after he inadvertently hit a rut in the ice and crashed into the leg of Oilers player Shawn Horcoff in overtime.
"It's unfortunate for us and for him, but it's just another thing we'll grind through," coach Davis Payne said. "When he gets healthy, we'll have him back in there."
McDonald is tied for the team lead with 17 points.
He missed the final 29 games of the Ducks' Stanley Cup run in 2003 because of post-concussion syndrome after missing seven games earlier that season after suffering a concussion.
"It's tough. Everyone kind of felt it when we saw [McDonald] go down in Edmonton," center Jay McClement said. "Hopefully, it's not real long-term. It is what it is. We can't do much about it. A lot of teams go through it."
Janssen and Colaiacovo have since returned from their injuries. Perron has been sidelined since Nov. 4.
"Nobody's going to feel sorry for us," forward Brad Boyes said. "We'll just regroup and hopefully get past it."
St. Louis hosts the Columbus Blue Jackets on Thursday.