Manager Arsene Wenger said Arsenal's Premier League title chances had suffered a major blow with Sunday's 2-1 defeat by Chelsea in the London derby.
Arsenal slipped to third in the table and six points adrift of leaders Manchester United after Chelsea came from a goal down to win with two strikes from Ivorian Didier Drogba.
Asked about its impact on a title bid that was firmly on course just a month ago, Wenger told a news conference: "It's a big setback today for us.
"When you add the four games we drew, and to lose today, we have to get back to winning games before we dream of the title.
"I feel the spirit is fantastic in the team. Today the players will be very disappointed because they gave everything to win this game and we go home without a point.
"We have to show our strength. At the moment we are six points behind Man. United and we have first to win our next game before we can speak about the title."
Wenger had no doubts about where his side, who have won only one of their last eight games, need to improve.
"We have to become stronger again defensively," the Frenchman said.
"It's a game we should have won, but we have shown lapses in concentration that we paid for."
Chelsea manager Avram Grant was cautious over whether his side, now second and five points behind United, could go on and win the league for a third time in four seasons.
"I don't know, but we will do our best to," said the Israeli, who replaced crowd favourite Jose Mourinho last September.
"Since September, we have had 80 per cent good games and we have scored a lot of goals. It's important.
"Today, we conceded a goal...but they responded very well."
The turning point came midway through the second half, when Arsenal lost defender and goalscorer Bacary Sagna to injury while leading 1-0 and Grant sent on Anelka and Juliano Belletti. Drogba's goals soon followed, confounding sceptics in the Stamford Bridge crowd.
Asked about his substitutions, Grant said: "I don't think any manager thinks it will succeed every time. Today it did, so I'm happy... We wanted to put more pressure on their defence."