I think that was more down to the wages, large loan fee and just coming back from doing his cruciate. This along with the fact that United's central defenders were Evans, Jones, Smalling, Blackett and McNair yet they were still making a large, risky investment in a striker. He's going into Chelsea with lower expectations, filling an actual need and though I don't know the specifics, I'm assuming he's not on £250,000+ p/w and they haven't paid £6mil to get him. Same player but it's apples and oranges in terms of the deals. United's one was low risk, high reward and in a postion they didn't need, Chelsea's is literally the opposite.
There were glimpses while he was at Untied, very rare but there's something still there. Season long loan as a 2nd/3rd choice striker with very low expectations at one of the best teams in the world under the best manager. If he can't regain form in that sort of environment, he's done.