head coach has provided two reasons as to why his side lost 3-1 at on Sunday.
The Blues fell behind at the London Stadium and after Carney Chukwuemeka made it 1-1, should have put Chelsea ahead but his first-half penalty was saved.
Chukwuemeka was forced off with injury at the break and replaced with Mykhailo Mudryk, .
Pochettino believes the mentality shift after Fernandez’s missed penalty, combined with Chukwuemeka’s injury, were key to Chelsea’s downfall in east London.
‘It was the frustration when we miss the penalty,’ Pochettino, who is still seeking his first win as Chelsea head coach, said after the match.
‘We were playing well and we didn’t get the reward. We start the second half a little bit with this belief until we started again to play.
‘Also the injury to Carney Chukwuemeka changed a little bit the shape of the team. Then we start again to dominate the game but again we concede.
‘We need to be more strong in this type of situation because we knew West Ham sit deep and counter-attack and set-pieces are really dangerous.
‘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.’
Speaking more generally, Pochettino added: ‘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. We create many chances and we did well in the first half. We should have won the game with our first half.
‘In the second half with another isolated action we concede again. To find the space to create clear chances [is difficult] but 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.’