I was also surprised with this news honestly. Let's say Kovacic didn't have much playing time there then it would have made sense. However there was no such situation either. He has been a regular in Chelsea starting lineup for years. I would have said that it was maybe due to his performance. But it was a horrible season for Chelsea so everyone was inadequate in performance.
At least they shouldn't have sold him to a rival team in the same league. Manchester City have newly lost Gundogan and it didn't even take them a long time to fill his place. They have filled the place with a really solid midfielder like Kovacic. I don't know what Boehly is really planning to do with this.
Kovacic's move to Man City definitely a perfect signing for them. After losing Gundogan, Kovacic can fill his position. Kovacic is undoubtedly a good player, but, Gundogan may possess a slight edge in terms of overall performance.