Jose Mourinho reveals the reason why he benched in-form Diego Costa

Despite being in top goalscoring form, Chelsea striker Diego Costa was left on the bench to face Schalke tonight.

Chelsea have enjoyed a blistering start to the new Premier League season with wins over Burnley, Leicester City, Everton and Swansea City, but their Champions League campaign didn't follow suit after a 1-1 draw with German outfit Schalke at Stamford Bridge. Cesc Fabregas grabbed his first Chelsea goal in the first half, but Klaas-Jan Huntelaar rescued a point for Schalke to earn a share of the spoils.

The Blues' impressive Premier League form has been spearheaded by new signing Diego Costa. The £32m arrival from Atletico Madrid has hit seven goals in his first four Premier League games so far, but the Spaniard was controversially left out of the Chelsea side tonight.

Instead, manager Jose Mourinho decided to hand a second Stamford Bridge debut to Blues legend Didier Drogba, but the Ivorian couldn't mark his full return with a goal as Chelsea were left frustrated in the opening Group H tie of the competition. Many Chelsea fans were wondering why in-form Costa had been left out of the squad, and Mourinho revealed the reason why he made his decision to leave Costa on the bench until the 74th minute tonight.

"Diego Costa is not in a condition to play three matches in a week," the Portuguese told Sky Sports. "He has problems at this moment. When he plays one game he needs a week to recover to be in condition to play in the next game. We play now in midweek – he can’t do it. If he starts [in this game] maybe I have to change him after 25 minutes."

Costa has been plagued by injuries over the last six months; he famously played through a hamstring injury in the Champions League final for Atletico Madrid against Real Madrid, but lasted only nine minutes as he withdrew from the game to save further injury ahead of Spain's World Cup campaign.

Then in Brazil, Costa looked like a shadow of his usual self, looking lethargic and off the pace, clearly carrying another injury. It's therefore little surprise to hear that he isn't 100% just yet, but Chelsea fans will be hoping that he reaches peak physical condition in the coming weeks and months as the Blues look to mount considerable challenges for silverware in multiple competitions.

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