While playing for the Baggies and Toffees, Lukaku has scored 33 goals in 71 appearances—an impressive strike rate for any striker, let alone one who is barely out of his teens.
Is Romelu Lukaku good enough to start every week for Chelsea?
Lukaku is yet to find the back of the net for Chelsea, though, and with Mourinho opting to bring in a proven goalscorer from elsewhere this summer, it seems his chances of becoming the first-choice front man at Stamford Bridge have reduced significantly.
The problem Lukaku faces is that, in the big games, when it has counted most, he has never quite turned it on in the way Mourinho would have hoped.
He was ineffective coming off the bench for the opening matches of 2013-14, while Lukaku's penalty miss in the Super Cup shootout against Bayern Munich outlined a player lacking the composure to carry the pressure that comes with playing elite football.
It's no different on the international stage, either. Take this summer's World Cup.
Lukaku started Brazil 2014 as Belgium's first-choice striker after team-mate Christian Benteke suffered serious injury near the end of the Premier League campaign, ruling him out.
It was the ideal opportunity for Lukaku to demonstrate his ability in front of a global audience of millions, playing himself into Mourinho's plans in the process.
Yet, with everything in place, he failed to live up to his billing.
After some poor displays early into the tournament, Marc Wilmots replaced him with Divock Origi, forcing Lukaku to watch on from the bench.
And if he isn't given another loan move this season—or sold—it's a view he will have to get used to at Stamford Bridge.
He will argue on the contrary no doubt, but if the World Cup has shown us anything about Lukaku, it's that a role coming off the bench is perhaps in his best interests right now.
It's a given that he must earn the right to be regarded a starter at Chelsea, but that's not only it. Appearing as a substitute for his country, Lukaku played his best football in Brazil.
Against the USA in the last 16, Belgium bombarded Tim Howard's goal for much of the game, struggling to find a breakthrough in the 90 minutes.
When the game eventually went to extra time and Lukaku was introduced into the action, he turned things around in his team's favor.
It wasn't just his physicality that shook the American defence, but also his movement and within three minutes Kevin De Bruyne had scored to put Belgium ahead.
Right on the interval of extra time, Lukaku struck the killer blow to double Belgium's lead with a goal that proved his only strike at the World Cup and significantly the winner against the USA.
Indeed, it was interesting to witness how Lukaku played with more confidence and conviction against Jurgen Klinsmann's side.
His 30-minute display may have been a cameo in comparison to others, but it was the most important of any other player, including Eden Hazard, De Bruyne and Dries Mertens.
Lukaku was a bully in the way he needs to be every week for Chelsea; however, at just 21, he is yet to add that very quality to his repertoire.
Football is about making an impact at big moments, and Lukaku's biggest one for Belgium this summer has come as a substitute against a tiring defence and one not completely au fait or ready to deal with his physical attributes.
When he joined Chelsea in 2011, there was talk of Lukaku being the next Didier Drogba. He matches up to his former team-mate in body shape, but he isn't clinical enough yet.
By allowing him to blossom with more cameo displays at Chelsea, Lukaku's development will benefit for it.
We've seen how he performed with less pressure on him at the World Cup, so why not at Chelsea?
Lukaku wants to be playing every week now, though, and that's a considerable problem for his manager and his employers. Do they allow a youngster to dictate his position at the club without doing much to warrant it, or do they cut their losses and potentially lose out on a world-beater for the future?
If Mourinho can convince Lukaku otherwise, it's an arrangement that will suit everyone in the long term.
Garry Hayes is Bleacher Report's lead Chelsea correspondent. Follow him on Twitter @garryhayes