He will be able to talk to Nou Camp chiefs in January and sign a pre-contract just six months before his Blues deal finishes.
The 31-year-old, who joined Chelsea from Russian side Anzhi in 2013, had previously indicated he could stay.
But after clocking up more than 330 appearances for Chelsea, scoring over 50 goals, there is a strong possibility he will depart this country in the summer.
After initially being left out by new boss Frank Lampard, he has forced his way back in and had become a regular.
Chelsea were reportedly ready to offer him a fresh two-year deal with a third as an option that could be taken up by the club.
Lamps is keen to get Willian signed up but will not be party to the contract negotiations.
He said: “I can’t get involved in the money, that’s up to the club, but I think everyone sees I’m relying on him and I love him at this football club.
“When I looked at him at the start of the season, I saw him being huge for me and the way we want to play.
“The off-the-ball work he’s done is an absolutely outstanding example for Callum Hudson-Odoi, Christian Pulisic and other young wingers who think the only part of the game is going past people and crossing or shooting.
“He does the other side brilliantly, with absolute humility, so he’s been a joy and one of the experienced players that’s really helping the young boys.
“I want to see him happy, I know him and he’s a happy, really easy-going lad who loves his football.”
But the decision to put on hold his contract renewal talks could force Lamps to put the Brazilian back on the sidelines as he plans for the future.
Barca had a £55million bid rejected by Chelsea in 2017.
Spanish newspaper Mundo Deportivo claim: “Willian is in no hurry to sign the Chelsea renewal proposal.
“The Brazilian is very comfortable in London and feels valued, but he has never hidden his illusion of signing for Barca.”
Juventus are also ready to join the race for Willian, they claim.
The Italians have brought in a series of players on free transfers for big wages, which they did with Aaron Ramsey in the summer.
Both clubs are set to talk to Willian’s representatives from January 1.
The Brazilian will see how things unfold before deciding his next move.