Diego Costa
Diego Costa still struggling with fitness problems, according to Jose MourinhoGetty

Chelsea manager Jose Mourinho has urged Diego Costa not to worry about his current goalless run but to concentrate on improving his fitness levels.

Costa, a £32m summer signing from Atletico Madrid, struck seven goals in his first four appearances for the club and still maintains an impressive ratio of 11 goals in 12 appearances this season.

The Brazilian-born Spain international has however failed to find the back of the net in his last three appearances and has been overtaken by Sergio Aguero in the Premier League's top goalscorer charts.

Costa struggled to deliver any sort of impact as Chelsea succumbed to their first defeat to the season against Newcastle United on Saturday 6 December but his manager is not worried by his striker's current woes in front of goal.

"For me, the problem is not the goals – they will come as a consequence of his condition," Mourinho said. "He has to improve his condition. He was injured and had many doubts because he went a long time without training.

"Obviously that must have an effect on a player in the way he runs, the way he moves and in his co-ordination. I don't think there is any problem but he has to recover."

Costa's recent struggles have seen him score just twice in his last seven appearances for Chelsea, form not helped by a persistent hamstring problem that has fuelled a club v country debate between Mourinho and Spain boss Vicente del Bosque.

Mourinho has previously opted to rest the player for some of the Blues' Champions League group games and may choose to do the same for the visit of Sporting Lisbon on Wednesday 10 December, with the club's place in the knockout stage of the competition already assured.