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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.
What makes the bundesliga less tasking? Don't they have the same 90 minutes of playing time as the premier league? And is it not with a complete team that they play against? If we are talking about Sancho let's focus on him alone and stop comparing leagues in this case. We all know that Sancho is a good player and for the past 2 seasons he hasn't really gotten the playing time that is required for him to produce his best. A player cannot give the best of his potential when he is not playing regularly. If Sancho can play in a team where he is guaranteed regular playing time, his best form will return.