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The squad depth can help against unexpected events so that if an injury occurs to a core player it does not affect the outcome of the game. Pep Guardiola does not have a good squad depth so it will be a bit difficult to rotate when his core players get injured and that is evident in how inconsistent their performances are this season. This is different from the previous seasons where the reserve squad they had was no less good than the core players so that even if the players were injured it did not affect the results of the match.
New players are indeed needed because De Bruyne and several other players are not in good form and maybe age factors also affect. So with the presence of several additional players will make Pep Guardiola have options in doing rotation and most importantly not affect the team's performance in the competition.
There has been an improvement in performance from most of Manchester City's main players in the last few games, so I think Pep Guardiola can rely on them. Without too many injuries, I think Manchester City's main squad is still better than PSG's main squad. The reserve players are not too bad, it's just that when the team is down, the main team will also perform badly.
Manchester City certainly has to maximize its opportunities to get several players in the market. They need a good midfielder and also a productive winger, of course this will help them increase their chances of winning in each match. The competition in the second half of the season will be more difficult, as will the remaining Champions League matches.