Thats for sure a big loss for Bayer Leverkusen that Moussa Diaby has left the team , and also sad for the german league because i loved to watch him playing.
But i think that Leverkusen will be investing the money they get or got for him good for another good player.
While i agree that Diaby was a very talented player that made the difference in a lot of matches for Bayer Leverkusen, i also think that he had quite a few matches where he showed pretty average or bad performances so he is still lacking the consistency in order to become a real world class player. Therefore i think this was a very good deal for Bayer Leverkusen even though his qualities will definitely be missed. They will get 55 Million € for him (+5 Million of possible bonus payments) and for that amount of money i am sure they can definitely find a proper replacement for Diaby i am pretty sure they already know how to replace him.