But Montgomery told The Journal: "Craig's been really unfortunate.
"When he first came in he didn't have the best of times. Whether it was the £9m (transfer fee Sunderland paid Hearts) on his head, I don't know.
"He was out of the side for a while with his broken arm and when he came back in he had a great period. Then he got the pin taken out of his arm, then his injury to his knee. When he came back the second time he did really, really well."