The midfielder has just undergone Achilles surgery after being injured in last week's friendly against New England Revolution.
Sarri said: "In Boston the pitch was not very good, but with an injury like that there isn't only one reason, there are probably a lot of reasons. We cannot say it was only the pitch.
"The injury is very similar to Callum's (Hudson-Odoi) injury, not the same but very similar. I don't know the timing, it's very difficult to say, four months, five months, it's difficult. Probably he will be able to have training with the group in five months, then he needs to be able to play at 100 per cent of his possibilities. I don't know exactly the timing but four months to six months.
"Loftus-Cheek was able to become a very important player for us, especially in an open match. He is useful in space for us. I am disappointed but it is useless to speak about him now.
"I am sure he will be able to return stronger than in the past. He will become for sure one of the most midfielders in Europe."