"James has unfortunately broken his leg," Levein told the clubs official website after the game.
"I really do feel for him. He has only just burst onto the scene in the last week or so and has performed remarkably well for one so young.
"Psychologically this will be a real blow for him, but he is mentally a very strong kid. I wouldn't have had the confidence to play him against Celtic if I didn't think he had great mental reserves and that will stand him in good stead when he is recovering from this injury.
"The break is in a bad area and it looks as though it is going to have to be pinned. You could ask me to guess when he will be back playing but I have really no idea. I do know though that it is a serious injury.
"He was in the team on merit and the evidence was there to show that in the game.
"It's a blow for the club because his energy was infectious. Joey Gudjonsson replaced him and had a great game because I think he felt under pressure.
"With Patrick Kisnorbo being injured at the moment it was a blessing for me to have someone like James to appear on the scene from nowhere.
"It's unfortunate and I just hope the lad gets back as quickly as possible."