The 20-year-old signed a five-year contract in August 2011 before spending the majority of his first season playing for the reserve team. He refused to hold the Champions League trophy, as he did not feel that he had contributed to the victory. After a promising preseason tour of the USA, Lukaku was sent on loan to West Bromwich Albion in August 2012.
Once there, he scored 17 goals in 20 appearances, 15 of which were from the bench. A hat-trick in their final game against Manchester United capped an excellent season, yet speculation over his future has lingered. A second loan spell has been suggested, but The Sun claims Lukaku wants to play for Chelsea.
There have been obvious comparisons made between the youngster and Didier Drogba, Mourinho's favoured front man from his first spell as Chelsea manager. Lukaku's performances for West Brom have proven that, alongside the physical similarities, he shares Drogba's lethal eye for goal.
Where he will fit in at Chelsea depends on the Blues' activities in the rest of the transfer window, but Lukaku's skill and strength would allow him to play up front in several different formations.
He played predominantly as a lone striker at West Brom in a 4-2-3-1 system that was similar to Chelsea's. The Blues struggled to get goals from their strikers in this formation last season, but Lukaku offers a different skill set to current forwards Fernando Torres and Demba Ba. His power and acceleration allow him to make bursting runs to get on the end of passes from the creative midfielders, although he will need to improve his offside awareness in order to continue as the lone target man.
Another option would be to deploy him as part of a twist on a 4-4-2. Sky Sports reports that Chelsea are apparently making inquiries about want-away Manchester City striker Edin Dzeko, and the Bosnian could make the perfect partner for Lukaku up front.
Dzeko's 14 goals from 16 substitute appearances in the 2012-13 Premier League highlight his qualities as a poacher. If Lukaku was to act as the big target man, laying balls off for Dzeko to finish, the pair could forge a world-class partnership.
If Mourinho continues his efforts to sign Edinson Cavani from Napoli as reported by ESPN, then Lukaku's chances in the Premier League are likely to be limited. However, cup competitions could offer an outlet for the youngster, with the opportunity to become the starting striker in both the FA Cup and Champions League.
Whatever happens in terms of transfers, Mourinho will need to make sure that Lukaku does not feel marginalised as he did in his first season, or Chelsea could risk losing one of their brightest young prospects.