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I dont know what player can currently replace him on that position with the similar quality. He has been the brains behind a lot of attacks of Man City in the previous season and without him this is gonna cause problems. We seen already against Sevilla that there were many problems in getting that direct ball for the goal.
So far it has been difficult to find a suitable replacement for Kevin De Bruyne at Manchester City, but it is not impossible.
Pep Guardiola is a coach figure who is able to optimize the appearance of a player under his care so that I think he can get a suitable replacement for De Bruyne in the future. Of course he needs time, but I think Pep Guardiola can.
Cole Palmer is one who will probably continue to improve under Pep Guardiola. He has a good game as an attacking midfielder, but Cole Palmer is not as good as De Bruyne. Cole Palmer will need time and hard work to reach his high level of play, but only if he is to stay a little longer.