Southampton manager Ronald Koeman said his side made too many basic errors in their 3-0 home defeat by Premier League champions Manchester City.
Saints, who dropped to third in the table, conceded three second-half goals despite City being down to 10 men for the final 16 minutes at St Mary's.
"We had the possibility to show our qualities and it was not good enough," Koeman told BBC Sport.
"Only two or three of the players reached the level that they can show."
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Yaya Toure, Frank Lampard and Gael Clichy scored the goals to secure City's triumph, with the visitors making light of being denied a clear penalty in the first half and also Eliaquim Mangala's second-half red card.
Koeman said he felt his side looked nervous during the opening exchanges and admitted they got punished for basic errors in the second half.
"You know that if you do some mistakes you are punished for that. It was not good enough and we have to realise that," he said.
"The difference was the quality. Chelsea and Manchester City are the two strongest teams in the Premier League and you have to be good.
"If you are not at your best and make some mistakes then they will punish you."
Southampton play Arsenal away on Wednesday and Koeman says his side must now focus on the challenge awaiting them at the Emirates.
"There is no time to be disappointed," he said "From tomorrow we have to prepare for Arsenal. Wednesday is a new opportunity to show our qualities."
Jack Cork missed Sunday's match because of an illness while fellow midfielder Morgan Schneiderlin is a doubt for Wednesday's game after being taken off at half-time with a muscle problem.
"Morgan had pain in his muscle," said Koeman. "It was not the same injury as before. Maybe it will be difficult for Wednesday."