His three seasons at Dortmund have been decent, horrific, and amazing, with the last widely credited to a specific manager with a style very different from Mourinho's. He might have the worst career Champions League record I've ever seen for a highly rated attacking player (4 goals, 0 assists in 38 matches). He is a very good signing at the price because his ceiling is very high (as shown by last season) but you really don't know what you're going to get with him. I would have taken him at Arsenal in a second, considering both risk and reward. But he is an ultimate high-risk signing that could go either way and having reservations about banking on that kind of player seems perfectly normal.