The Teessiders have been in pursuit of the Scotland international this summer but the Whites have made it clear that they do not want to sell.
McCormack has scored three goals in four Championship outings so far this season but still Mowbray feels he may be able to pry him away from Elland Road.
"I'm loathe to mention individuals, but McCormack is a player we have enquired about,” he told the Gazette.
"I think it is three in three he has scored now for them and he is a fans' favourite.
"It might be difficult for them to sell him, but we can only ask the question of clubs.
"I don't think it's over. Look at the bigger clubs - is the (Wayne) Rooney issue over, is the (Luis) Suarez issue over?
"Once the window shuts, he won't be signing. But I've got total respect for the club the boy is playing for and he is doing pretty well for them."