The Brazilian was the Blues' hero against Newcastle, scoring a stunning winner in a 2-1 victory.
The 30-year-old has been a long-term target for the Catalan giants – but says he is happy at Stamford Bridge.
Reports have continued to link Willian with a move to the Nou Camp but the winger says he hasn't seen them.
When asked about stories linking him away from Chelsea, Willian told BT Sport: "My future is here at Chelsea, I don't know what you're talking about.
"I didn't see the papers."
Speaking after the win, Sarri was then asked about the Willian situation.
The Italian claimed he did not know how long the winger has left on his deal – but he sees him as a crucial player to his plans.
"We all know very well, Willian is a very important player.
"As I said [Friday], Willian has to stay with us.
"For us he is very important.
"Willian, for us in this moment, he's a fundamental player."
Sarri also made it clear that he wants another defensive playmaker to cover Jorginho, after the departure of fans' favourite Cesc Fabregas.
Sarri added: "Today Jorginho was in trouble.
"On the bench we didn't have a player for that position, so we need [one]."
Sarry says he has "no idea" when that player – rumoured to be Nicolo Barrella or Leandro Paredes – will arrive.
The Italian also said he was not too worried about the lack of options in that position and that "the club know my opinion well, I need a player in that position."
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