Willian will wear the No.10 shirt at Chelsea in the 2019-20 season, succeeding Eden Hazard after the Belgian attacker left to join Real Madrid.
The Brazilian winger had worn the No.22 jersey ever since he joined the Blues in 2013, but has now swapped it for one that has been associated with Hazard for the last five years until his €100 million (£91m/$111m) move to the Santiago Bernabeu.
Meanwhile, Willian's previous number has gone to Christian Pulisic, who arrived at Stamford Bridge this summer from Borussia Dortmund, and 21-year-old Tammy Abraham has been given No.9 following his loan spell in the Championship with Aston Villa.
Mason Mount is also expected to play a role for the Blues as he reunites with coach Frank Lampard, having impressed under the former midfielder at Derby County last term.
The 20-year-old has taken 19, while fellow returning players Michy Batshuayi, Tiemoue Bakayoko and Kenedy keep the numbers they had before they were sent on loan - 23, 14 and 16, respectively. Reece James is 24 and Fikayo Tomori has the 29 jersey.
Kurt Zouma, who wore No.5 before being sent for spells at Stoke and Everton, is now 15, while Danish defender Andreas Christensen has traded 27 for 4.
Chelsea have promised a free exchange to fans who have already purchased next season's shirt with previous numbers on them from the club's official store. Those who bought them from other outlets will be offered a 50% discount on a new shirt.
👀 A closer look at this year's squad numbers! pic.twitter.com/yxAHGRwakd— Chelsea FC (@ChelseaFC) August 3, 2019
Chelsea begin their Premier League campaign next Sunday when they visit Old Trafford to take on Manchester United.
The London side finished third in the English top-flight last term and won the Europa League, but coach Maurizio Sarri left after just one season in charge to take over at Juventus.
Blues legend Lampard has stepped in to replace the Italian, but has not had much room to improve the squad as the club are currently serving a transfer ban.
Pulisic is the only new face, having been signed in January and loaned back to Dortmund to see out the remainder of 2018-19.