Exactly that is where Pep Guardiola got it wrong by not signing enough players last summer transfer window, this had resulted to their poor runs, though he had some few signings like return of Gundogan from Barcelona and signing of Sarvinho, I believe Pep Guardiola thinks that he has a formidable squad with depth not considering injuries that might hinder his squad, now a handful of first team players are out injured among them include Rodri, Ake, Akanji and Stone those players would have make a positive impact on the team had they been fit unfortunately Pep Guardiola didn't have perfect replacement for them and it obvious some of the squad players are getting old thus wouldn't play optimally players like Debruyn, Silva and Gundogan.
I think this is not entirely Pep's fault before he decided to recruit players, he must have discussed it first with the club management, not just carelessly recruiting the players he needed, especially Man City last season was hit by many accusations so they have to be careful in acting if they don't want to make a fatal mistake, bad luck is indeed no one knows Pep Guardiola must feel quite confident with the existing squad because he thinks he has mastered the game for years but it seems this season he has run out of tactics and his strategy is starting to be easily read by his opponents.