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 pounds 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 there were a few actions we didn't manage well and we concede," Pochettino told Sky Sports.

"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 76% 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.

"It was the frustration when we miss the penalty. We were playing well and we didn't get the reward," Pochettino said.

"Disappointed but this is only the beginning. We need to believe in the process. I think we are going to be stronger and for sure be really competitive."

For West Ham, it was a massively encouraging afternoon and a result that suggests they can avoid a season spent looking over their shoulders at the relegation places as they did last time.

James Ward-Prowse made an instant impact on his debut, setting up the opening two goals for Nayef Aguerd and Michail Antonio and even after Aguerd was sent off midway through the second half West Ham's resilience shone through.

"We hung in there at times, were resolute. Even at 10 men we stuck in and got another goal," manager David Moyes said.

"We stuck at it, did the basics well and got the three points."