He just returned from a bad injury when he joined Man Utd so that could of been a reason to his poor form plus the fact he was acclimatising to a new league. On his day he's far better than Costa and depending on the fee Chelsea may have got a bargain.

I remember Diego Forlan being absolutely terrible for Man U but then changed clubs and proved what a good player he was. Torres was another who had mixed fortunes. Brilliant for Liverpool but crap for Chelsea. Shevchenko too.

Class is permanent, form is temporary.