The last dozen players to wear the shirt have struggled to make a prolonged impact – including three club-record signings up front, but also a holding midfielder and a certain Dutch defender.
Here, the PA news agency looks at the “Curse of Khalid Boulahrouz”.
Pierre-Emerick Aubameyang, 2022-23
“It’s cursed, it’s cursed, people tell me it’s cursed!” said then-manager Thomas Tuchel last summer, adding: “There was not a big demand for number nine, nobody wants to touch it.”
Aubameyang was not deterred but, after arriving with a broken jaw sustained in a robbery at his Barcelona home and seeing his former Borussia Dortmund boss Tuchel sacked the day after his debut, he failed to establish himself under Graham Potter – he was not even included in the squad for the Champions League knock-out stages, despite scoring two of his three Chelsea goals in the group stage.
Romelu Lukaku, 2021-22
Despite 15 goals in all competitions, it was no surprise when he returned to Inter on loan. He scored 14 goals in the season while wearing number 90 at San Siro and has been left out of Chelsea’s pre-season tour of America.
Tammy Abraham, 2019-21
Jimmy Floyd Hasselbaink was the last inarguably Chelsea number nine but amid almost £250m spent to fill the troublesome shirt in the 18 years since his departure, academy product Abraham produced arguably the most successful spell with 15 league goals and 18 in total in 2019-20. Twelve more the following year earned a move to Roma under former Blues boss Jose Mourinho.
Gonzalo Higuain, 2018-19
The Argentinian’s loan from Juventus yielded five goals in 18 appearances. He was shunted to number 21 on his return to Juve and terminated his contract early the following season to move to Inter Miami.
Alvaro Morata, 2017-18
A then club-record buy for £60m, the Spaniard scored 15 goals in his debut season but fell out of favour, switching to number 29 and adding another nine goals before joining Atletico Madrid.
Radamel Falcao, 2015-16
The Colombian scored once in 12 games in an injury-hit season.
Fernando Torres, 2011-14
The then British-record £50m signing from Liverpool got the longest run at making the Blues’ number nine shirt work for him, wearing it 172 times across three and a half seasons.
He scored 45 goals, including a memorable Champions League semi-final clincher against Barcelona, and set up another 26 but several prolonged scoring droughts saw him move on to AC Milan on loan and then his boyhood club Atletico Madrid.
Franco Di Santo, 2008-09
Fourteen scoreless substitute appearances were all Di Santo had to show for his time in west London.
Steve Sidwell, 2007-08
The former Reading midfielder managed only 24 appearances, scoring once. A largely defensive player, he would have been a glaringly unlikely wearer of the number nine shirt had it not been for his immediate predecessor…
Khalid Boulahrouz, 2006-07
The versatile Netherlands defender was assigned one of the few available numbers after his £8.5m arrival from Hamburg. He lasted one season and 23 appearances in all competitions before a loan to Sevilla and a permanent exit to Stuttgart.
Hernan Crespo, 2005-06
The Argentina striker had already spent a season at Stamford Bridge wearing number 21, scoring 12 goals, and a year on loan at Milan before Mourinho recalled him and gave him number nine as a show of faith. He added another 13 goals and a league title but soon returned to his earlier employers Inter.
Mateja Kezman, 2004-05
Inheriting the shirt from Hasselbaink, Kezman failed to make a similar impact with just seven goals in his sole season.