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.
I wonder why Moussa Diaby could accept to play at Aston Villa, because next season Aston Villa will also only play in the Europa Conference League. While if Diaby stays at Leverkusen then Diaby will play in the Europa League and also, basically Diaby has a good performance in making passes and also goals. Although Diaby is basically only a winger and with the potential he has, then Diaby should be able to play at a bigger club of course. So indeed, with Diaby departure in this transfer window, I believe Leverkusen might be very badly affected later on even if only for some time.