He said: "There were one or two after him when Sunderland signed him and they did well to get him.
"One or two eyebrows were raised at the fee they paid for him but he is certainly justifying that, not that he has to. He is scoring some important goals.
"He has done really well. He looks a handful. He puts himself around, works very, very hard, and he has got the goals his performances are warranting."