Romelu Lukaku has left Chelsea to return to Serie A on another season-long loan deal, this time with Roma, and it means he remains one of the Premier League's great enigmas.

Lukaku's latest Stamford Bridge departure comes 12 years after he first joined the Blues from Anderlecht as an 18-year-old, and it follows two summers on from him becoming Chelsea's then-record signing for £97.5m.

While his move to Roma is only on loan, he is already cumulatively the most expensive player ever, with his career transfer fees totalling nearly £300m.

The 30-year-old was more prolific - in a goals-per-game sense - than Wayne Rooney, Robbie Fowler and Jermain Defoe in the English top flight, scoring at a rate of 0.44 goals per game across 278 appearances.

Yet some critics suggest Lukaku was not a success in English football.

So was he underappreciated or, considering sheer potential and money spent to acquire him, a precocious talent unfulfilled?

Lukaku's Premier League rise

It began at Chelsea.

Plucked from Anderlecht as a teenager, with his time on the Blues' payroll overlapping that of Kevin De Bruyne, Lukaku played 15 times across all competitions over his first two seasons for Chelsea without getting off the mark.

Instead he impressed on loan at West Brom, netting 17 times in the Premier League - more than any Chelsea player in that 2012-13 season. The haul included a famous hat-trick in a 5-5 draw against Manchester United in Sir Alex Ferguson's last match as a manager.

Lukaku's stock continued to rise on loan at Everton in the following season and he signed permanently at Goodison Park for what was a club record £28m in 2014.

The forward repaid that with 68 goals in 141 Premier League games, becoming the Toffees' top scorer in the competition and only the fourth player to score 80 career goals in the competition before turning 24 - after Michael Owen, Fowler and Rooney.

The 25-goal haul Lukaku netted in 2016-17 remains his highest total in a single league campaign. It was the first time an Everton player had scored more than 20 goals in a league season since Gary Lineker in 1985-86.

After criticising the Toffees for lacking ambition and then turning down a new contract, Lukaku was signed by Jose Mourinho's Manchester United for an initial £75m.

"Lukaku is a different player now to when Mourinho sold him at Chelsea. He's a better player and his record is sensational," said former Everton and United defender Phil Neville at the time.

"He plays on the shoulder of defenders, he likes to run over the top of defenders," added ex-Everton midfielder Leon Osman. "I don't think I've seen a better one-on-one-type finisher with the goalkeeper than Lukaku."

Lukaku's hot streak continued, scoring 10 goals in his first nine games in all competitions to better a club record set by Sir Bobby Charlton, who managed nine in nine, and the Belgian ended his debut season at United with 27 goals, including 16 in the league.

Ronaldo Koeman, Everton boss at the time, was not happy with a 2017 interview in which Lukaku suggested the club lacked ambition

Serie A sabbatical

Lukaku was unable to maintain that form and his downfall at Old Trafford coincided with Mourinho's, who later said the forward "needs to be loved".

Criticised for his weight - and subsequently diagnosed with a digestive problem by Inter's medical staff - Lukaku went on a 12-game drought in 2018 and found himself out of the team under Mourinho's successor, Ole Gunnar Solskjaer.

Speaking later to the LightHearted podcast, Lukaku suggested he, Paul Pogba and Alexis Sanchez were being made scapegoats at United.

He was bought by Inter for a club-record £74m and entered the most productive period of his club career, equalling Brazil great Ronaldo's club-record 34 goals in all competitions in his first season.

He also became the first player to score in 10 successive Europa League games, though the forward's own goal in the final - having earlier scored a penalty - helped Sevilla win 3-2.

Lukaku got the silverware he sought the following year - his first trophy since the 2010 Belgian league title with Anderlecht, where he won the golden boot aged 16 - when he scored 24 times as Antonio Conte's side won Serie A for the first time in 11 years.

"He always had immense physical and athletic qualities, but during the two years together we saw him grow even more in terms of presence on the pitch, teamwork, and composure in front of goal," Conte told L'Equipe.

Two years after leaving the Premier League, however, Lukaku was back at Chelsea, becoming a team's record signing for the third time in his career.

"I came here as a kid who had a lot to learn, now I'm coming back with a lot of experience and more mature," Lukaku said.

Lukaku first joined Chelsea as an 18-year-old in 2011

Tuchel's tactical conundrum

"It's a slam dunk," said the Athletic's Italian football writer James Horncastle on 5 Live's Football Daily podcast at the time.

Blues striker Chris Sutton added: "If there was a criticism of Chelsea last season, it was that they possibly weren't ruthless enough and now they have a guy in who is an absolute glutton for goals."

That never materialised. Lukaku's return yielded eight Premier League goals in 26 games and he struggled to fit into boss Thomas Tuchel's system.

During a 1-0 win at Crystal Palace in February 2022, Lukaku touched the ball only seven times - the lowest number any player has managed in a 90-minute Premier League appearance since at least 2003, when that data was first recorded.

"Of course it's not what we want and what Romelu wants, but it's also not the time to laugh about him and make jokes about him," said Tuchel afterwards.

"How can you pay that much money and not play to his strengths?" asked former Arsenal striker Ian Wright on Match of the Day.

Lukaku was also not helped by an interview conducted earlier in the season with Sky Sport Italia, in which he said he wanted to return to Inter in the near future.

That wish was granted when he returned to the Nerazzurri on loan last season, while his latest temporary move to Roma reunites him with former Chelsea and United boss Mourinho.

Regarded as world class?

Lukaku came on as a substitute during Inter's Champions League final defeat by Manchester City in June

Perhaps it is in Serie A where Lukaku feels his qualities are most suited.

He scored 10 league goals last season, as he was blighted by injuries, but has 76 goal contributions in 97 Serie A games, meaning a goal or an assist on average every 99 minutes over three seasons in Italy, compared with every 132 minutes in his Premier League career.

That is complemented by a remarkable 75 goals in 108 appearances for Belgium, and Lukaku feels his record does not get the credit it deserves.

"When people talk about Lewandowski, [Karim] Benzema or [Harry] Kane, they call them world class," Lukaku said during Euro 2020.

"But when it comes to me, it's just considered good form. That motivates me to work harder and get stronger.

"I belong in that list of world-class players."