Scans have revealed McKay has a minor tear in his meniscus, as well as bone bruising, following the Blues' 21-point loss to Collingwood on Saturday night.
The one-time Coleman medallist's match ended after just 10 minutes, following his knee knocking into teammate Sam Walsh.
McKay tried to come back on the ground, spending an extended period on an exercise bike, but was instead ruled out early in the second quarter.
Carlton say McKay will be monitored this week, failing to rule him out of this Saturday's clash with Gold Coast.
But meniscus injuries tend to be tricky to recover from, with McKay to be in some doubt to play at all again this season.
Interim Carlton coach Josh Fraser was unable to shed any light on McKay's injury, after several commentators were left confused and questioned the key forward's condition.
"Those ones can sometimes look worse than what they appear," Fraser said.
"I don't know. I haven't received any more information on it."
But Carlton dodged a bullet with midfielder Adam Cerra who was cleared of a hamstring injury following scans.
Cerra, who has a history of soft-tissue injuries, didn't play any further part in the game from midway through the third quarter due to hamstring tightness.
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