His best attributes aren't really fit for Sarri's system. Crawling the ball up the pitch and Hazard coming deep for it at times, etc... Fast transition football is a rare thing with Sarri.
And that's what Kovacic does best - defensively reliable, pacey and comfortable on the ball, but his biggest strength is picking the ball in his own half and absolutely blasting past people to open up the pitch for his teammates.
Zidane's Real was perfect for that, and that's why Kovacic improved a lot at Madrid - both for "Real B" (side that basically won them the league with Kovacic, Isco, Morata, James, Vasquez, Danilo, Nacho, etc., who won a LOT of games while "Real A" was rested for CL - Kovacic had more minutes than Modric that season in La Liga, for example - Modric was injured a bit but yeah), and then last season where he (Kovacic) was basically a starter until his injury.
What he does need to work on, if he wants to be truly world class, is off-ball movement.
I may have been spoiled by Modric, who I consider possibly the best in the history of the game (yes) at opening up passing lanes for his teammates, but Kova's off-ball movement pales in comparison.
TLDR; I don't think he will work at Chelsea under Sarri. He can't take the next step in that system, and Chelsea won't get a world-class CM to do what Sarri asks of them in that tight, closed system.
And whether he becomes a world-class CM, I do not know. Potential is there though, and he could become one in a different system.
(for example, I think he would fit Liverpool and Liverpool's system would fit him like a glove)
