The young winger went through with an operation on his Achilles injury. He will now go through rehabilitation, while there is possibility he will have another surgery.
Sarri said during his press conference: "The operation of Callum went the right way. I think he will be back here in one or two days.
"Of course he needs time now, but Callum is a very strong young man - physically and mentally. I am sure he will be able to return better than before.
"It depends [but] for sure, three months.
"Then we have to see the condition, because you can be able to play but we need a player at the top. The technical period is three months, no less."