Julen Lopetegui called on his team to improve after they were beaten 3-0 by Nottingham Forest.
Goals from Chris Wood, Callum Hudson-Odoi and Ola Aina blew away the Hammers at the City Ground.
After the game, Lopetegui pinpointed Edson Alvarez’s red card, which came for a second yellow card after a rash tackle on Anthony Elanga, as the pivotal moment of the game.
To make matters worse, the Mexico international initially walked towards the corner flag after receiving his marching orders, rather than towards the tunnel.
It is another setback for the Irons coach, who only last week saw his side beat Manchester United 1-0.
Lopetegui has won only three of his opening 10 Premier League games, representing a hugely underwhelming start to the season after a big summer spend following the departure of former manager David Moyes.
“We can and we have to do much better,” he said. “You can prepare the team but the players have to take the right decisions and we have to play with eleven men.
“It's one of the situations we can avoid because there was no danger of this. We had a lot of players in our box.
“It is a big advantage for the opponent above all because there is a lot of time in front of you. You are losing the match so it was a bad moment for us and a very good moment for them because they wanted to damage us on the counter-attack.
“That's why in the second half we tried to wait a moment. We were not able to because we suffered the second goal and it was different. That's why I repeat that today is a hard night for us.
“That's why we have to say first of all sorry for our fans. We are very disappointed today and we have to use these hard moments to push forward and to look forward to the next challenge.
“We know that, even if you are showing control [in the early part of the match], you have to avoid the counter-attack, you have to defend the crosses better.
“It happened that they scored a goal in this action and after the goal it was more difficult for us.”
