The veteran is set to sign a new deal with Real Madrid.
Former Real midfielder Kovacic told AS: "Modric's career is unrepeatable, we are lucky to have him. Only when he retires will we be aware of everything he has done and is doing.
“It's too much (to play at Modric's age), but I've always expected great things from myself. Injuries and health are a big factor, so I have to work on that.
"We'll see, Luka is a phenomenon who is difficult to repeat. Real Madrid's oldest player who, moreover, continues to be an important factor. I don't know if it will be possible to replace him here in Croatia, it will take a lot of players, a whole team."
On City boss Pep Guardiola, he also said: "You learn a lot from Pep, from the details he explains to you. The way he analyses and prepares matches is truly top level.
"It's an honour to have arrived at a club like this and with a coach of this calibre. The first season in a new club is always the hardest, but they accepted me very well and Pep helped me a lot."