Manuel Pellegrini says goalkeeper David Martin will continue in goal for West Ham, but admitted it was a "difficult decision" to drop Roberto.
Martin made his Premier League debut against Chelsea at Stamford Bridge on Saturday - keeping a clean sheet in a 1-0 win - and he will retain his place for Wednesday's away match against Wolves.
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Hammers boss Pellegrini was delighted with how the 33-year-old performed on his West Ham bow, but insists he had thought long and hard about replacing Roberto.
"Of course, it was a difficult decision because the goalkeeper is a special position," Pellegrini said. "Martin had not too much experience in the Premier League.
"I think, in some moments, the team needs a reaction and, unfortunately for Roberto, he was not doing well. It's my duty to try to see what we can improve.
"I don't think that was the only reason why we played well against Chelsea though. I think there was commitment from the players. We were aggressive in the way that we played and we improved a lot in comparison with other games."
Despite being dropped from the starting XI for the second game in a row, Roberto has been backed by his manager to rediscover his best form.
"Roberto is a player with a strong mentality," Pellegrini added. "He has a lot of experience and a career behind him.
"I hope that he will keep working, forget what's happened, and he will continue to rediscover his normal performance, because that wasn't normal for him to make mistakes."
'Haller needs time to adapt'
Forward Sebastian Haller was also among the substitutes against Chelsea, watching on from the sidelines as Michail Antonio started up front.
Haller has failed to score in his last six appearances but remains West Ham's top goalscorer this season, and Pellegrini believes the Frenchman will soon be back to his best.
"All the players that come from different leagues need time to adapt," Pellegrini said. "I think that Sebastien started very well, scoring some goals, but after that he was struggling.
"It wasn't just his performance; he wasn't the only one whose performance wasn't normal. The team was not playing well.
"He understands that he will continue being an important player and I am absolutely sure that soon he will be the Sebastien he was at the start of the season."
'My pressure is my challenge'
West Ham's first victory away at Chelsea since 2002 ended an alarming eight-game winless run and sees them climb up to 13th in the table.
Pellegrini found himself under increasing pressure at the London Stadium prior to the win, but the Chilean insists he remains calm and relishes the challenge.
"The pressure against Chelsea was exactly the same pressure as for the first game of the season. It was exactly the same when we beat Manchester United," he said.
"My pressure is my challenge, my challenge is to grow this club, to arrive to a big goal this season. We know we have a better team and we will continue fighting.
"You feel highs and lows. You must be calm in those moments when you are not winning and not think you are the worst manager. When the team is winning, it is the same, it is not because you are the best manager."
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