Adaptation to a new coach is something players do take time even if it isn't such a long one, but they don't get it right almost immediately, I believe that with the right kind of coach, most players are going to do much better than they have been and that's why pep like your sighted is and was considered a good Coach. Injury prone player are actually a challenge to a team at some point and they're some times been sold out especially when their from drops so bad and the team isn't even willing to cope or continue with any of such.
For Barcelona flick will make a good coach but they will have to give him the time he needs and not to be impatient with him at the very earliest stage of his stay at Barcelona. I believe that Barcelona will get back better again in the new season.
I completely agree with you that adapting to new coach takes time even for talented players. It is unrealistic to expect immediate results but with right guidance players can reach new heights. Good coach like Pep Guardiola is prime example of this as you mentioned. On the other hand injury prone players can be liability for teams and sometimes it is necessary to move on from them. I think Xavi will be great coach for Barcelona but he needs time to implement his vision and strategy. Team must be patient and not expect instant success. With time and support I am confident that Barcelona will bounce back stronger in the new season.