Sunderland boss Paolo Di Canio insists his players must be fitter for the fight for Premier League survival.
He said: "I know exactly what they can give me in terms of energy.
"It is a short time but we are capable of building their strength, resistance and most importantly their attention, because the goals came from mistakes."
Di Canio continued: "It is not the fittest team in the world but we are going to work and give them more energy in the next few days and weeks."
In his first game as Sunderland boss, controversial appointment Di Canio cut an animated figure in his technical area as he bellowed instructions to his players.
Sunderland took the lead just before half-time when Chelsea defender Cesar Azpilicueta turned into his own goal following John O'Shea's flicked header from Adam Johnson's corner.
But the Blues responded straight after the break. Fernando Torres, who replaced the injured Demba Ba at half-time, supplied Oscar, whose shot ended up in the net via deflections from Sunderland keeper Simon Mignolet and defender Matt Kilgallon, who was debited with the own goal.
Chelsea got the winner 35 minutes from time when Branislav Ivanovic reacted quickly to divert David Luiz's long-range shot out of the reach of Mignolet.
The defeat means Sunderland only stay out of the bottom three on goal difference, with a crunch clash against local rivals Newcastle to come on Sunday.
Di Canio, who was once again questioned about his political views in his post-match media conference, said: "I was more than happy with the first half. They went through the strategy very well.
"Chelsea dominated when they kept the ball but they do that against other sides at Stamford Bridge. We were well organised and the players ran very hard. The second half was difficult because we lost two quick goals and you can get blurry if you concede these goals."
Di Canio admits he has had to take a different approach with the Black Cats than he did in his last job with League One Swindon Town.
He added: "There is a difference. At Swindon you are preparing to play Accrington and then you are against Eden Hazard and Oscar, top footballers in the world.
"Before at Swindon I only used to prepare the game to win. I still only prepare the game to win but the strategy is a bit different because the opponents can keep the ball.
"That can be a big change but if you are a good manager like I am you can do it."