I'm sure they have a lot of options to put in the midfield. They can evaluate one of them and make up for Gundogan's absence. As for the team captain, I think it is no longer as important as it used to be. İlkay leaves, someone else becomes captain and Manchester City continues to win, I don't think anything will change.
Indeed, there are many stock players that Pep Guardiola can rely on in the midfield. However, Pep Guardiola believes that Gundogan departure is a loophole that must be fixed immediately by bringing in other players. That was evident after Kovacic was brought in where he would be the player to close the gap left by Gundogan.
Manchester city is a big team and they don't depend on one player. That's because there are many players who have better quality so that it will make it easier for Pep Guardiola to build a squad without losing balance after Gundogan leaves.
Pep is a coach who thinks a lot about his opponents and his team and creates a team that can dominate the opponent and achieve victory in any situation.
He thoroughly prepares each game and talks a lot with his players, explaining to them all possible situations in the game and how they should react. No coach prepares as thoroughly for games as he does.
Pep demands a lot from his players, above all pressing and ball possession, and for such a game his players must be fully physically and mentally prepared.
Gundogan was a very important player for such a Manchester City game, but apparently Pep has already found a replacement for him in Kovacic.
Manchester City will be the main favorite to win the Champions League this season as well.