Chelsea are willing to extend Romelu Lukaku's contract at Stamford Bridge despite doubts over his role under José Mourinho and the expected £32m arrival of Diego Costa.

The Belgium international has barely featured for Chelsea since signing for a fee rising to £18m from Anderlecht three years ago and spent the past two seasons on loan, at West Bromwich Albion and then Everton. He faces another challenge next season to convince Mourinho he merits a starting place but, with only two years of his contract remaining, Chelsea will offer a new long-term deal unless they receive a substantial offer for the 21-year-old after the World Cup.

Lukaku turned 21 only on Tuesday and his recent successful loan spell at Everton, when he scored 15 goals to finish as the team's leading goalscorer, was the first time he has made over ten Premier League starts in a single campaign. He started 29 league games , as Roberto Martínez's team finished fifth to qualify for the Europa League.

The Everton manager has made a priority of signing Lukaku on a permanent basis but Chelsea want in excess of £25m for the forward, a figure that is likely to prove prohibitive at Goodison Park. Tottenham Hotspur have also been linked with the powerful striker, as have Atlético Madrid as part of a deal for Costa.

Chelsea are keen to protect their investment in Lukaku, however, and unless they can make a profit on the young striker this summer will look to secure him on an extended contract while his development continues. That would increase the possibility of a third season away from Stamford Bridge on loan, a prospect that would also appeal to Martínez, although Lukaku has made no secret of his desire to play Champions League football and finally make an impact at Chelsea.

Lukaku had a more productive Premier League campaign than Chelsea strikers Samuel Eto'o, Demba Ba and Fernando Torres, with Mourinho frequently lamenting a lack of penetration in his attack. All three could depart Chelsea this summer but that would not guarantee more playing time for the Belgian, who has had a frosty relationship with Mourinho since he returned to the club from Real Madrid. He would also be behind Costa in the pecking order at Chelsea next season if, as expected, the Brazilian-born Spain international completes his move from Atlético.