I just look at how they did in the CL before he took over and how they did after and to be honest afterwards they didn't look such a good side in the CL. He came 3rd in his last season, 3rd in a league with basically 2 teams is pretty poor, Dortmund player wise we head and shoulders above everyone bar Bayern.
If we're after him, we're aiming pretty low IMO, they didn't sack him for no reason, I remember seeing Dortmund and thinking they looked very beatable compared to the side they were, even Liverpool knocked them out after needing 3 goals twice in a game in the Europa, pretty poor show IMO, remember thinking how brittle Dortmund looked compard to before he took over. Is this really the kind guy we can attract now, a 2nd rate manager with no real record of success? If so we're in big trouble.
I'm not interested in any manager that can't build a team capable of defending, you'll never win anything of note without a good disciplined defence, top teams are built from the back.