Arsene Wenger bemoaned defensive lapses after Arsenal squandered a two-goal lead for the second time in four days.
"It was frustrating because we had the chances," said Wenger. "We were caught in too comfortable a position. We did the difficult thing, going 2-0 up.
"But then we lacked defensive urgency."
Arsenal's solidity was a feature of their play in the opening weeks of the season - they did not concede in their first three Premier League matches.
But they were far from solid at the back at Emirates Stadium against Fulham, and that is a worry for Wenger and assistant manager Steve Bould - particularly after Schalke, a below-par league display against Manchester United last weekend, and a crazy 7-5 Capital One Cup victory at Reading.
Wenger, though, was keen to stress he was pleased with his players' efforts against the Cottagers.
"In the second half we gave absolutely everything and I have to give credit to the players," said the Frenchman. "In the last three games what they have done physically is amazing. Even at 3-2 down we didn't give up and continued to go forward and we had the chances to win the game."
The performance of forward Olivier Giroud, whose two goals took his tally for the season to six, was highlighted.
"Giroud was dangerous all along and he was the positive of the day," added Wenger. "He answered some questions that were raised."