The Gunners knew they would go 11 points clear of United with their first win at Old Trafford since 2006.
The response, therefore, must have been pretty disappointing.
It led to a first league defeat since the opening day of the campaign, allowed United to force themselves right back into contention and means Liverpool are now just two points adrift of the Gunners in second spot.
"It was a bit of a missed opportunity," said Vermaelen.
"We were eight points ahead of United going into the match and had the chance to make it 11.
"Of course it was disappointing that we lost. We wanted to win the game."
The solace, such as it is, comes from the knowledge when Arsenal suffered a similarly shattering loss against Tottenham last season, they immediately embarked on a 15-match unbeaten run away from the Emirates Stadium that swept them back into a Champions League spot that seemed certain would elude them.
Now Arsenal will look to show the same resilience, and prove wrong the doubters who are back out in force.
"We are still top of the league," said the Belgian.
"We need to keep the confidence we've had over the last couple of weeks.
"There's no reason for that to change dramatically. We're still confident we can win games.
"There is still a long way to go."
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