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Barcelona are interested in getting Gondogan in this summer's transfer window, especially with free agent status where Gundogan contract with Manchester City will end in June. I think his certainty of staying with Manchester City will not come true after he postponed the delay to sign the contract proposed by Manchester City.
Maybe you're right, Gundogan seems interested in moving to other teams including Barcelona considering he hasn't decided his future with Manchester City at the moment. He is old, but he is an influential player.
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The rumors have been on for sometime now, with the season now over and City have three more trophies added into their trophy cabinet, the players can now make a decision on their future, so that the club can make adequate provision for replacements. Gundogan and Bernado Silva are very important players for City, but i'm pretty sure that if they leave, Pep would be able to identify who he wants as replacements, and the club would try to get them. Kovacic is almost certain to join City this summer and there would be more reinforcements coming in, City wouldn't struggle next season even if they lose these two players.
Of course it will be replaced, but the quality will definitely not be the same.
Gundogan was captain at Manchester City, he has influence in the squad but that will only end soon if he doesn't extend his contract. I've also heard about Kovacic going to Manchester City, but either way the quality won't be the same.
Pep Guardiola has shown in the past that he's very good in replacing members of his squad immediately they leave his club and he does it so efficiently that the player's absence wouldn't be felt no matter how important he was to the team's progress. He did it after the departure of Vincent Kompany, Yaya Toure, Sergio Aguero, and David Silva .
However I think IIkay Gundogan still has a lot in the tank to give out when it comes to delivering top performances for ManCity but time will tell if he'll remain in the club to do more for them or be allowed to leave