Would you prefer being in easy Champions League group or having difficult opponents? I don't understand how facing easy teams is NOT good. You can rest your key players and secure promotion to knockout phase soon. Manchester City had luck with a draw this year.
The money and injuries part are two important reasons but it is not the key ones. Injuries could happen even in training and money can be collected from other important games.
Playing vs easy teams will make the players more confident in a negative manner which may have an impact in the next games. Man City is a professional team indeed but facing some challenges is required in every step. We will see what will happen next