Mauricio Pochettino called on Chelsea fans to "believe in the process" after his side slumped to a 3-1 defeat away to West Ham United in the English Premier League on Sunday.

The Argentine former Tottenham Hotspur and Paris St Germain manager has taken on a big challenge at Stamford Bridge, trying to fashion a new identity for an expensively-assembled squad.

Eight players have been signed for around £325 million ($414 million) since he arrived in the summer as Chelsea try to re-invent themselves after finishing 12th last season.

One of them, Moises Caicedo, whose fee from Brighton could break the British record, was given a debut off the bench but his impact rather summed up Chelsea's day as they remained on just one point from their opening two games.

His rash tackle inside the penalty area on Emerson in stoppage time gave Lucas Paqueta the opportunity to wrap up the points for West Ham from the spot and he made no mistake.

Chelsea have won just one of their last 14 Premier League games and have now lost four London Derbies in a row but Pochettino is certainly not about to start panicking.

"That's football. Teams like us, we need to get the right balance. I think today (Sunday) there were a few actions we didn't manage well and we concede," Pochettino said.

"Overall it is this type of game that is difficult to explain. You had the possession, you create and maybe you were the better side but in the end you lose."

Chelsea had 75 per cent of the possession at the London Stadium, had 17 attempts at goal and 706 passes compared to West Ham's 225.

But apart from a very well-taken goal by Carney Chukwuemeka they had nothing to show for it with Enzo Fernandez, still the British record signing, missing a penalty to make it 2-1.