"Romelu was always clear that in his mentality and his approach he was not highly motivated to come to a competitive situation at Chelsea," he explained to the Daily Mirror (via the London Evening Standard).
"He wanted to play for Chelsea, but clearly only as first-choice striker—and at a club of our dimension it's very difficult to promise a player that status."
From the outset, that was always Lukaku's problem—the key to Mourinho's assessment being his motivation.
As an 18-year-old, he had enjoyed elevated status at Anderlecht, and when he arrived at Stamford Bridge for £18 million, he expected something similar.
But this is English football. The clubs are bigger, the competition fiercer and, more often than not, reputations count for very little.
To be a success at Chelsea, Lukaku needed to show the club he could cut it at the highest level.
Impressing on loan with West Bromwich Albion and Everton was a good place to start, but the impression he gave was that he warranted an immediate selection upon his return to West London.
It doesn't work like that, though Lukaku seemed to think it should.
The signs were there when Andre Villas-Boas was sacked as Chelsea coach in 2012.
Villas-Boas, Zenit Saint Petersburg's current manager, was the man who brought Lukaku to Chelsea. With Didier Drogba and Fernando Torres already at his disposal, however, Villas-Boas selected Lukaku just nine times.
"Right from the beginning [my relationship with Villas-Boas] was weird. I could not stand how he was treating me. I received no explanation," Lukaku reflected in May 2012, per Sportwereld (h/t the Daily Mail).
"In training, he put me on the right or left wing. In training matches, I was a substitute with a shirt to keep swapping teams. I really learned nothing!"
What Lukaku should have learned is that he had to earn the right for his manager to play him in the way he wanted, to feature as a key player at Chelsea.
He wasn't playing in the Jupiler Pro League anymore. His reputation, considerable price tag or not, counted for very little, especially when his competition for a starting berth included players who had won everything in football, while he had achieved little.
His approach was wrong from the outset. Where was the desire to continue his football education, to go out and prove himself as a Chelsea player?
Lukaku may well go on to have the career he believes he will in football. He has shown glimpses of that potential at The Hawthorns and Goodison Park, delivering to some degree on the talent we know is there. Whether his career scales the heights he so desires with a major European club, though, remains to be seen.
If it does, his character needs a drastic overhaul.
His antics throughout his Chelsea career didn't befit an elite player. He wanted to play every week, yet, as Mourinho was eager to explain, the motivation to compete was never there.
When we consider Cristiano Ronaldo, Lionel Messi, Frank Lampard, Zinedine Zidane, Drogba—all players who are or have been among the world's finest in their careers—that's an accusation never pointed at them.
In contrast, Lukaku wanted it on a silver platter.
It's why the Belgian is an ideal fit with Everton. A once powerful club of the English game is trying to find its place in the modern era, and to do it, there will be times when the manager and board need to give in to the whims of their star players.
Everton need Lukaku as much as he needs them. It's the perfect match in that regard, and Lukaku stands a better chance of flourishing simply because he's more inclined to have it his own way.
Back in London, where Chelsea are fighting for the Premier League and other major trophies overseas, it's different.
What Chelsea and any other elite club require is a group of players willing to compete and deliver. They've signed one of those in Diego Costa this summer, and despite the money Roman Abramovich has thrown at the cause this past decade, it's those principles that have delivered success.
Whether it's immaturity or arrogance, Lukaku hasn't shown he is that type. Not yet.
He was always doomed to fail at Chelsea.
Garry Hayes is Bleacher Report's lead Chelsea correspondent. Follow him on Twitter @garryhayes