I don’t really know why Manchester City didn’t perform well in the match, maybe it’s because of De Bruyne's injury. I think Pep Guardiola has to look for a replacement for De Bruyne because he will be going for surgery and he won’t be coming back soon. Let’s see how Manchester City will perform against Newcastle in their next match. I know it won’t really be easy for Manchester City.
Pep did not start to look for replacement after that match but he actually did it after the last season Champions League final. He does know the importance of De Bruyne with Manchester City and does not want to play without him. However, if we watched Manchester City in the past season, in some weeks, Pep did not use De Bruyne and he tried to replace the Belgian player as well as gave him more motivation to come back, playing simpler but more dangerous.
The departure of some of the best players means they don't have a similar replacement yet. Gudongan, making the midfield line has no attack creator other than De Bruyne. If Bruyne is injured then Man City are like losing a creative person. I think against Sevilla they lost 2-0, it was difficult for them to win. Unfortunately, several goal-ready chances failed.
Replacing De Bruyne and Gundogan will not only player replacements but also tactical operations as well. They are both very important players with Manchester City in last 6 years or longer and they provided creativeness for attacking as well as balance between attacking and defense in Manchester City midfield.
Pep with his genius vision and idea for tactics, will be able to replace those players but in the meantime, if he can not find good player replacements, he will adjust the club tactics to deal with it.