Still the Blues’ club record signing, the Belgian will be reunited with his former employers on loan after struggling during his first season back at Stamford Bridge. The £97m signing could not find any consistency in west London and eventually fell out of favour with boss Thomas Tuchel.
Whilst his performances were of varying quality, the blame can’t solely be laid at Lukaku’s door with a number of other big-name strikers having failed at the club after assuming the number nine shirt at Stamford Bridge during the Premier League era.
Take a look at the full list of Chelsea’s number nines over the past 30 years, sorted by goals per game average, and see how the Belgian frontman compares.
Jimmy Floyd-Hasselbaink - 0.49 goals per game
Arguably the most successful number nine in Chelsea history, Dutch striker Jimmy Floyd-Hasselbaink was signed from Atletico Madrid in 2000. He went on to assemble a stunning strike rate at Stamford Bridge with 87 goals in just 177 appearances in all competitions for the Blues.
Of course, it was no surprise to see Hasselbaink a success with Chelsea with the forward having previously sparkled with Leeds United. Following his goal-laden spell at Stamford Bridge, he went on to represent Middlesbrough for two seasons before retiring with Cardiff City.
Gianluca Vialli - 0.48 goals per game
Joining from Juventus ahead of the 1996/97 season, Vialli was a huge name across European football and despite arriving at Stamford Bridge towards the end of his career, still turned in some excellent performances.
The Italian scored 40 goals in just 83 outings before becoming the Blues’ boss in a player-manager role following the sacking of Ruud Gullit. He even scored on his final appearance for the Blues in a victory against Derby County.
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Mark Stein - 0.43 goals per game
A signing from Stoke City in the early 1990s, Stein was another to put together an extremely respectable strike rate. He scored 25 goals across his first two seasons in West London before then returning to Stoke City on-loan.
The South African born striker was then loaned to Ipswich before joining Second Division side Bournemouth, where he continued to display his eye for goal.
Tammy Abraham - 0.38 goals per game
The occupant of the number nine shirt before Lukaku’s arrival, Tammy Abraham was afforded an opportunity in the first-team under Frank Lampard with the Blues under a transfer embargo. He repaid that faith with 18 goals in his first season before following that up with a further 12 during the following campaign.
However, he was sold to AS Roma to make room for Lukaku and he ironically had a better season than the Belgian with 27 goals to his name as the Italian side won the Europa Conference League under Jose Mourinho.
Romelu Lukaku - 0.34 goals per game
Having never quite proven his worth at Chelsea during his first spell at the club, it looked like the perfect time for Lukaku to finally write a name for himself at Stamford Bridge after being brought back last summer and handed the number nine shirt.
Despite a bright start, his performances and goal-scoring tally quickly tailed off with just eight Premier League goals to his name ahead of a switch back to Inter Milan. It now seems certain that the Blues will only lose money on their record signing.
Alvaro Morata - 0.34 goals per game
In an ironic turn of events, Chelsea could have resigned Lukaku in 2017 but elected for the services of Alvaro Morata instead - the Spaniard proving as disastrous.
Arriving from Real Madrid, he struggled to settle and quickly returned to Spain only 18 months after his £60m arrival with a loan to Atletico Madrid. The La Liga giants signed him permanently before a further loan move to former club Juventus and he is now in an unfortunate situation of not knowing where his future lies.
What went wrong for Alvaro Morata at Chelsea? Have your say in the comments...
Mick Harford - 0.34 goals per game
Before being able to splash huge fees on forwards, Chelsea looked in the lower leagues for their recruits and that was certainly the case with the signing of Mick Harford. The forward was brought in from Luton Town ahead of the first Premier League season but he could only score ten goals in 29 outings for the Blues.
He was not even at Stamford Bridge for the whole season as Sunderland swooped for the two-time England international’s services in the following March.
Hernan Crespo - 0.31 goals per game
Despite having an excellent record in all for Chelsea with 25 goals in 73 appearances, it appeared that the number nine shirt was too heavy for the Argentine international to bear.
He scored 12 goals in 29 appearances donning the 21 shirt before a season-long loan to AC Milan. He then returned with the number nine and scored just a further 13 goals in 42 appearances.
Gonzalo Higuain - 0.28 goals per game
Having developed a fearsome reputation at Napoli and Juventus, Higuain arrived at Chelsea with his powers having waned. He spent the first-half of the 2018/19 campaign on-loan at AC Milan before then arriving at Chelsea in January.
The Argentine enjoyed a promising start with three goals in his first five Premier League outings but could not extend that record having netted on just five occasions in six months whilst at Stamford Bridge. It goes without saying that the Blues did not take up the permanent option in the deal.
Fernando Torres - 0.26 goals per game
A former British record transfer, Fernando Torres had developed the reputation as one of the most electric forwards in European football during spells with Atletico Madrid and Liverpool. Chelsea swooped for his services on a memorable deadline day in 2011 but he was simply not able to live up to the price tag - or number nine shirt.
It took 14 appearances for him to find the net for Blues whilst his highest Premier League tally at the club was just eight. He holds cult hero status at Stamford Bridge due to some of his performances in Europe - a famous goal at the Camp Nou a standout moment - but it is hard to argue that he justified the £50m fee that took him to Chelsea.
Tony Cascarino - 0.17 goals per game
A Chelsea striker for three seasons in all, Cascarino only inherited the number nine shirt for one season once set numbers were enforced. It wasn’t the season that he would have wanted with just four goals in all competitions throughout the campaign.
He found his goalscoring boots again with then Ligue 2 side Marseille, scoring 68 goals over two seasons for the French giants.
Mateja Kezman - 0.17 goals per game
A signing from Dutch side PSV Eindhoven, there was hope that the Serbian would have been able to reignite his partnership with Arjen Robben. However, he never got going with Chelsea having scored just seven goals throughout the 2004/05 season.
Kezman spent just one season in West London before signing for Atletico Madrid. He went on to represent the likes of Fenerbahce, Paris Saint-Germain and Zenit St Petersburg.
Radamel Falcao - 0.08 goals per game
A perplexing move at the time and one that looks even more confusing seven years later, Falcao joined Chelsea on-loan following a thoroughly underwhelming spell with Manchester United the season before.
He was even worse at Stamford Bridge with constant injuries limiting him to just 12 appearances for the club. The Colombian’s only goal came in a 2-1 home defeat to Crystal Palace.
Is Radamel Falcao one of Chelsea's biggest ever flops? Give your verdict in the comments...
Chris Sutton - 0.05 goals per game
A prolific forward at both Norwich City and Blackburn Rovers, Sutton moved to Stamford Bridge for a huge £10m fee and endured a woeful season as the Blues’ number nine.
He scored just two goals throughout the 1999/00 season - only one of those coming in the Premier League - before attempting to reignite his career north of the border with Celtic.
Steve Sidwell - 0.04 goals per game
Whilst the vast majority of the Blues’ number nines in Premier League history have been out-and-out strikers, the same can certainly not be said of Steve Sidwell.
A midfielder by trade, he arrived from Reading and was surprisingly given the shirt number. He only went on to make 25 appearances for the Blues throughout a short stay at Stamford Bridge - scoring one goal - before being jettisoned for Aston Villa.
Khalid Boulahrouz - 0 goals per game
Seeing Sidwell don the number nine was weird but the sheer fact that Dutch centre-back Boulahrouz also claimed the shirt was downright baffling. Arriving from German outfit Hamburg, he made just 23 appearances for Chelsea.
He was soon on the move once as he joined Sevilla for the 2007/08 campaign before a move to Stuttgart.
What are your memories of Khalid Boulahrouz in a Chelsea shirt? Have your say in the comments...
Franco di Santo - 0 goals per game
The man to claim the title as arguably Chelsea’s worst number nine of all time, the Argentinian was an obscure arrival from South American side Audax Italiano.
He did not score a single goal in 16 appearances for the Blues before then going on to frustrate for Blackburn Rovers, Wigan Athletic, Schalke 04 and many more. Somehow, he is only 33 years of age having recently represented Turkish side Goztepe.