Sarri said: "I think Mateo will be very useful for us. He's a very technical player.
"In this moment, he has to work. I think he's not ready now for 90 minutes, but maybe, tomorrow, for the first 30 or the last 30 minutes he could be used.
"I expect a lot from him because, for me, he's a very great player. In my opinion, he can improve more."
Kovacic has been earmarked to play on the left of Sarri's three-man midfield, but the Chelsea coach believes the 24-year-old can also play on the right and centrally.
"I think he can play in any of the three positions in midfield," Sarri said.
"He's a very good offensive player, but he has improved in the defensive phase in the last two years in Madrid."