London - Chelsea manager Thomas Tuchel has no doubts his team can sort out their problems quickly if they focus on the details after a 3-2 defeat at West Ham United at the weekend saw them slip from first to third in the Premier League.

"There is no need to question the big picture and no need for major doubts," Tuchel told the club's website. "In my opinion ... it's about details. That's why I think we can sort it out straight away.

"Nobody wants to make mistakes but it won't help if we just say, 'Hey, don't make mistakes anymore.' We need to be better in decision-making in crucial moments ... to be able to have clean sheets or concede a maximum of one."

The German added that he had made a mistake by bringing Romelu Lukaku off the bench at half-time in place of the injured Kai Havertz, saying the Belgian forward was lacking match fitness.

Lukaku did not play for over a month after being sidelined by an ankle injury, and featured only briefly against Manchester United and Watford.

"It's my fault. I don't think he is ready for 45 minutes physically but still we decided to bring him on for his height because we lost Kai," he added. "Kai had a crucial role in defending set-pieces, which is a huge threat from West Ham."

Chelsea next face Russia's Zenit St Petersburg in their final Champions League group stage fixture tomorrow. Chelsea have qualified for the knockout stages but are tied on 12 points with Juventus at the top of Group H.