The Scottish international sustained the injury during training last Friday and will have surgery on his knee in London later this week.
"It's horrendous to lose a player of James' calibre," McLeish told bcfc.com.
"I feel gutted for him because I really did feel he was coming into excellent form and getting back to his best, so it couldn't have happened at a worse time for him or for us.
"I think this season would have been a huge season for him because last season was his first full season (of playing regularly) in England and it would have strengthened him physically and psychologically as well.
"To lose him at this stage is shame for James obviously but it's hard on us as well."