Manchester City need to improve their concentration, manager Manuel Pellegrini said after his team's 2-1 home defeat by West Ham.
City won their opening five matches but have suffered back-to-back defeats by Juventus and the Hammers this week.
"In the last two games we didn't play with the concentration or intensity we had shown in the first five games," said the 62-year-old Chilean.
"We have 96 points left to play for - there's a long time to go."
City lost their first game of the season when Italian champions Juventus fought back from a goal down to win 2-1 in their Champions League opener on Tuesday.
Pellegrini's side were aiming to return to winning ways against a side which had already earned away wins at Arsenal and Liverpool this season.
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But they missed the chance to move six points clear at the top of the Premier League table, slipping to their first top-flight defeat in 12 matches.
"We must concentrate more because two balls decided the result," Pellegrini said.
Hammers loanee Victor Moses drilled in a low 20-yard shot and Diafra Sakho hooked in from six yards, before City's record signing Kevin De Bruyne fired in to halve the deficit.
"I'm frustrated as they were three very important points at home," Pellegrini added.
"They won their two away games before this game against Arsenal and Liverpool so I knew it would not be easy but we conceded two easy goals and that allowed them to be in front.
"We never believed we had won the title after just five games - we just always focus on the next game."
Winning away run is a dream - Bilic
West Ham's perfect Premier League away record after beating Arsenal, Liverpool and Manchester City is a dream, admits Hammers manager Slaven Bilic.
"It is a surprise to us as well that we have beaten Liverpool, Arsenal and Man City," said the 47-year-old Croatian.
"Nine points from the first three away games sounds like a dream, not a plan. There's nothing wrong with a dream."
Former West Ham defender Bilic, who replaced Sam Allardyce in the summer, guided his team to a 2-0 win at Arsenal on the opening weekend, then to a 3-0 success at Liverpool.
But they recorded arguably the biggest shock by beating a City team which had scored 11 goals, conceding none, in their opening five matches.
City dominated the game, controlling 72% of possession and attempting 27 shots at goal, but were unable to find a way past the Hammers who are now three points behind them in second place.
"We deserved to win," said Bilic. "City played much better in the second half and it was almost like The Alamo.
"But I have told my players to be proud, we deserved it, and we kept the ball when they let us.
"When City are losing and start to play faster and apply more pressure, it is hard to cope with. That's why I'm so proud - all credit to my boys."
'There were two Winston Reids'
Hammers centre-back Winston Reid drew praise from Bilic - a former Croatia defender - after being instrumental in helping his team clinch their fourth win of the league season.
The New Zealand international, 27, made the most clearances (20) and interceptions (five) during the match, while he also won back possession for his side (nine) more than any other player.
"Winston Reid was always in the right place - it was like there were two Reid brothers on the pitch!" said Bilic.