Actually before coming to Chelsea is form already declined and he did better for us than Manchester United.  His stat in the loan spell at Borrusia dortmund wasn't that good compared to his past stat the club. Also, the Bundesliga is less tasking than the EPL, how many players can you think of that couldn't perform well at the Bundesliga but did well in the EPL? But in the other way round there are quite some examples.

Don't speak as if Chelsea was the reason he declined whereas he was having a better performance there, trying to get back on track.