I don't think Manchester City will be shocked as they lost to Celtic(3-4). This is just a preseason match for them, a testing stage for most coaches to test the capability of their young players.
Yep. Why Man City must be shocked with the result? They only used most the players of second tier squad in the match. It totally doesn't reflect the strength of Man City squad. If they play with full squad and they play seriously, I am 100% sure they will win the match. Moreover, they have nothing to be achieved on the match, so they only enjoy the match and don't seriously think to win the match. Well, don't be so serious with a pre-season match!!

Big clubs will actually lose and win their most matches. When the main league match begins, they adjust their seat and sit up to give a fantastic performance because of their very match counts, it will not be taken for less preparedness to test players and match formation.
Not every big club but some big clubs experienced it. It actually depends on the coach, whether they will play seriously in the match or not. If they won't play seriously, they will mostly use second tier players and never order the player to win the match. Some clubs may not show their strength during the pre-season matches. It is also a part of their tactic to win the next season competition.