Apert from adapting to a new coach, Barcelona is getting choked with alot of injuries and once a player gets an injury there's a decrease olin form which my take Time to recover apert from recovering from the injury sustained in the first place..
Adapting to the new coach might not be as expected seriously., take man City as example , although a good coach will get his team to adapt with in a very short period of time and Pep is a good coach , Flick is also a good coach so I think we should expect fast adaption.
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.