Chelsea manager Jose Mourinho believes Fernando Torres' inability to score goals regularly is due to low self-esteem. The Portuguese tactician also stressed that the striker is not surplus to requirements at Stamford Bridge.

The 30-year-old has struggled to replicate his goal-scoring form for the Blues since making the switch from Liverpool in January 2011, scoring just 20 times in the Premier League so far despite making over 100 appearances.

The Spain international has made some glaring opportunities for the west London club, most notably against Manchester United in September 2011. Torres also missed a chance from two yards in this year's pre-season friendly against Slovenia's NK Olimpija Ljubljana.

However, Mourinho insists that the former Atletico Madrid starlet is a 'very useful' player for Chelsea as he enables him to deploy and implement various formations and tactics.

"Fernando is a good player and will not be sold. Maybe the self-esteem in terms of goalscorer is not at the top. All eyes are on him and when he misses a goal, everybody remembers," the former Inter Milan and Real Madrid manager explained.

"But his work for the team is fantastic and because of that we can play with two of the forwards at the same time, no problem. We can play Nando and Didier or Nando and Costa – like we did in Vitesse Arnhem on Wednesday when he played from the side."

"Maybe we could even play both through the middle. When we are not winning matches, sometimes we play with three defenders and two strikers. Nando will always be a choice and a very useful player for us," he added.

Mourinho has purchased Diego Costa from Atletico and also re-signed club legend Didier Drogba to bolster his side's chances of winning trophies next season.

The 51-year-old stated that the Brazil-born Spanish international was identified as a summer target in January.

"We needed to transform our squad a bit and especially the striker and midfielder position. We identified clearly the profile of the players we needed. With Diego we always had the feeling from six or seven months ago that we could ­possibly get him," Mourinho continued.

"We decided it probably would be good for us not to buy a striker in January ­because we knew we could get THE striker in June. So that was a very good decision by the club to wait for the striker we wanted," he concluded.