Willian says Callum Hudson-Odoi can be one of the world's best players if he stays at Chelsea.
Hudson-Odoi submitted a transfer request after public declarations of interest from Bayern Munich, which have been branded unprofessional by Chelsea boss Maurizio Sarri.
Reports on Monday suggested Chelsea have rejected Hudson-Odoi's request and have no intention of selling prior to Thursday's closure of the transfer window.
Willian believes Hudson-Odoi has great potential, which he can fulfil at Stamford Bridge.
"Of course we want him to stay. We don't want him to leave," the Brazilian said.
"He can become one of the best players for Chelsea, one of the best players in the world, in the future. He has to stay.
"He's only 18, but he can improve a lot to become one of the best players."
Sarri started Hudson-Odoi – who has played for Chelsea since the under-eights and is under contract until June 2020 – for a sixth time this season in Sunday's 3-0 FA Cup fourth-round win over Sheffield Wednesday.
The Blues boss was rewarded as Hudson-Odoi netted his second goal for the club, after which some fans sang "we want you to stay".
Defender Antonio Rudiger believes it would be a "pity" if the 18-year-old leaves Chelsea and has called on the Blues to make him stay and sign a new contract.
Rudiger admitted earlier in January that Hudson-Odoi had asked him about life in Germany.
The Germany international, who joined Chelsea from Roma in July 2017 and began his career at Stuttgart, said: "What I can say is clear, Chelsea have to make him stay. It will be a pity if this young talent leaves us.
"Chelsea need to give him a new contract."
Willian added: "We tell him all the time to stay here.
"We know how important he is for this club and for us and we just want him to stay."
Hudson-Odoi has been repeatedly linked with a move to Bayern, who have reportedly tabled a bid of £35million, while there has been speculation Chelsea are prepared to match the Bundesliga champions' financial offer.
Sarri's decision to play Hudson-Odoi on Sunday may be an indication that Chelsea will not be prepared to allow him to leave.
However, the game time may have come too late in Hudson-Odoi's eyes.
Hudson-Odoi's six starts have come in cup competitions and the Europa League. He counts a total of five substitute appearances for the Blues this season and two goals.
He is yet to start in the Premier League; three substitute appearances are the sum total of his appearances in the top flight and the first of those came at Watford on Boxing Day.
Chelsea play at Bournemouth in the Premier League on Wednesday, with the transfer window closing a day later.
It remains to be seen if Hudson-Odoi will still be a Chelsea player by the time the Blues face Huddersfield in the league on Saturday.
