Jimmy Calderwood told the Daily Record: "Scott Severin got injured in a tackle early on at Motherwell and tried to battle through it, which says a lot for the boy.
"Maybe it would have been better if he had come off immediately as he will be out for between four and six weeks.
"It's a massive blow because he has been one of our more important players in the past. But he is a very honest lad and will admit he has not been performing lately.
The Dons boss is also concerned that Steve Lovell could also be set for a spell on the sidelines after the striker had a scan on his knee last week. The striker has missed the previous three matches with the problem, and results of the scan should be revealed today.
"Obviously, I am hoping the scan doesn't show any lasting problem with Lovell either," said Calderwood. "Hopefully, though, [Kevin] McNaughton and [Michael] Hart will be back for the Hearts game."
McNaughton was the target of some speculation at the weekend, as his agent John Viola sent a story around the Scottish press, but Aberdeen-Mad does not want to give a mouthpiece for someone who we believe to be an obnoxious cancer on the game.