Clark is optimistic that Thorpe will return to action for the Blues this season.
"Tom has had an early scan which has shown significant damage to the ankle," Clark told the club's official website.
"There is still quite a lot of swelling there so we can't put a definite timescale on it, but we can say it will be a minimum of four weeks so that will rule out all of February. Potentially it could be longer but we're waiting to hear more news from Manchester United, who are continually assessing him.
"We can't cancel the loan because we have a contract and obligation to Manchester United, but we're hoping the lad can make a speedy recovery because we'd like to keep him if at all possible."
Birmingham travel to Charlton Athletic on Saturday.