Schürrle was Mainz's primary goal scoring option and most important player so it does matter. Real, Barcelona, Chelsea etc. also expect their star player(s) to put up good stats so it goes for Schürrle/Mainz as well. At Leverkusen he wasn't the main man but still, they could expect their second option on offense to score an important goal or at least assist one when they played one of the 2 best sides in Germany, but Schürrle didn't deliver enough.