(Reuters) - West Ham United's freefall in the Premier League standings in the past few weeks is proof that teams cannot afford to be inconsistent and take their foot off the gas in a highly-competitive league, manager Manuel Pellegrini said on Thursday.
West Ham were fifth in the standings last month after six games, which ignited debate over whether Pellegrini's side could qualify for European competition next season if they finished in the top six.
However, one point from their last three games saw the London side slide to 11th and Pellegrini said a lack of concentration in the league could easily kill a team's momentum.
"It's tough for every game. Maybe (if we had beaten) Crystal Palace here at home... we would have been sixth or seventh in the table, but maybe after one game you can be four or five positions back," Pellegrini told reporters.
"Every game you must play as if you're playing against the best team. It's why I don't only talk about West Ham, there are big teams who for whatever reason cannot be consistent.
"You have 20 teams who all have money to get good players. That's why it's so important to play 100% concentrated, because if not, it's very difficult to win the game."
West Ham host promoted Sheffield United, who beat Arsenal to move up to ninth, on Saturday and Pellegrini praised his counterpart Chris Wilder, whose side have been unbeaten in four away games this season.
"They're a consistent team," the Chilean manager said. "Having come from the Championship... they've a difficult style -- direct football, which we can prepare for.
"I think that Sheffield United reflect his (Wilder) mentality. It's clear in the way they play, it's absolutely English football -- strong players who work a lot over 90 minutes. They work to win the first and the second ball."
Full back Aaron Cresswell is back in contention after missing the 2-0 defeat at Everton but defender Winston Reid and winger Michail Antonio will not be involved.
"Compared with last weekend, Aaron Cresswell worked the whole week without any problems," Pellegrini said.
"Winston Reid continued his recovery and will probably will play the next game with the Under-23s on Wednesday. Antonio hasn't recovered, he has a long injury at the moment."
(Reporting by Rohith Nair in Bengaluru, editing by Pritha Sarkar)