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Manuel Pellegrini admits West Ham are missing injured goalkeeper Lukasz Fabianski.
The Hammers have not won a match since Fabianski suffered a hip injury at Bournemouth in September which will rule him out until the new year.
Second-choice Roberto has performed competently in his absence, but was unable to prevent defeats by Crystal Palace and Everton and was slow to get down to Lys Mousset's equaliser in the 1-1 draw with Sheffield United last weekend.
Pellegrini is convinced Roberto is not to blame for West Ham's downturn in results, but acknowledges it is "impossible" not to miss a keeper he feels is among the best in the country.
"Of course, all of us saw the performances of Lukasz last season and this season. He is a great goalkeeper. Maybe the best goalkeeper in the league," said the Hammers boss.
"We must be fair with Roberto. He is a good goalkeeper. He made very good saves in the games he played.
"But, always, to lose that kind of goalkeeper is important for everyone.
"I don't want to link that we don't have the results because Lukasz is injured. It's impossible not to miss him. But you cannot link the results to just one player.
"We have a squad. Roberto has quality. That's why we got him and he is not performing badly.
"That is not the reason we are not winning games. You must work with the players."
On Saturday West Ham host Steve Bruce's Newcastle, who have registered just two wins from their opening 10 matches.
"They've played a lot of difficult games against the big teams," added Pellegrini.
"They're a team that have had good results playing away, especially when they beat Tottenham. Chelsea beat them just 1-0, in a very tight game.
"Maybe they had some defeats but when they had one player less. They are a good team with a manager who knows his work, and good players who know the Premier League."
