I really like Kovacic, but suggesting that he made a serious contribution to four Champions League's is a total stretch, as he played almost no knockout games.
For his first win he played 34 minutes in the CL knockout stages, mostly in the first leg of the last 16 match. He wasn't in the squad for the final.
For his second win he played 6 minutes of knockout football in the quarter finals, none of it in the final.
For his third win he did start 2 out 7 knockout games, but again didn't play in the final.
For his fourth win he may actually have played more, but he was injured for the later knockout games, and came on for just 10 minutes in the final.
You can't compare with City last year. Firstly because it would only rule out a handful of fringe players, as most played significant games. But also because four CL titles is something very few players have, and I doubt he'd ever compare his record to the likes of Xavi, or Sergio Ramos.
It's absolutely no slight to point out that he was a squad player at Madrid when they were arguably the best team in the World, but was nowhere near a key player.