"He's recovering after his operation," McLeish told Sky Sports News.
"He's had a pin put in his leg so we're looking at six to nine months absence.
"It's very disappointing for Liam first and foremost and of course it's a crushing blow for the club.
"He's had a good season. He came back from a previous injury about a month ago and was playing really well. He looked like he was really on top of his game.
"The agony is that he'd passed the threshold for another booking because I had kept him out of a couple of games and his discipline has been excellent.
"It's a shame, but we've got to get on with it.
"We've got three game to go which will hopefuly take us to the Premier League if we can play to our capabilities."