Gundogan was pretty much the driving force in their team and they played their best stuff when he was at his best, and I was really surprised by some of the comments on the internet that since his back injury, when he missed a whole season, he's not the same. He basically was as good last season as ever. Of course, injuries are still a huge concern.
Oh and I don't want to sound as I know it all, I'm actually just translating what German experts were saying (just to name the most well-known in UK, Honigstein was full of praise for him in 14/15).
But we are slightly off topic. :)