Mateo Kovacic is likely to make his Chelsea debut off the bench when Arsenal visit Stamford Bridge in the Premier League on Saturday.
The Croatia midfielder was not involved in last weekend's 3-0 win at Huddersfield Town after signing on loan from Real Madrid on deadline day.
Cesc Fabregas also sat out the opening weekend and will miss the Arsenal match with a minor, "unusual" injury close to his knee.
"I think Mateo will be very useful for us. He's a very technical player," Sarri told reporters.
"In this moment he has to work. I think he's not ready now for 90 minutes, but maybe tomorrow for 30 minutes he could be used.
"I think he can play in any of the three positions in midfield. He's a very good offensive player, but he has improved in the defensive phase in the last two years in Madrid.
"I expect a lot from him because, for me, he's a very great player. In my opinion, he can improve more."
"I am not worried about Morata. Morata is a very great player," Sarri said. "I think that, in this moment, he needs only to gain confidence.
"He needs to score as all the strikers in the world do. He needs to gain confidence, but he's improving so I'm not really worried about him."
