Chelsea sweating on key man ahead of Newcastle FA Cup tie
The Chelsea forward limped out of last night's 2-1 Carabao Cup defeat to Arsenal with a hamstring problem.
Willian is now being assessed ahead of Sunday's fourth-round FA Cup tie.
Conte said: "The injury was decisive.
"Don't forget Alvaro Morata was out, and I was forced to play with Ross Barkley. He's a really good player, a great prospect, and is really important for Chelsea, but he needs a bit of time to understand our football."
Willian told manager Conte that the injury was not serious after the match.
"Willian said he felt a problem in his hamstring, but he doesn't think it's serious, and I hope he's right," said Conte.