Their night was made even worse as jubilant Lens supporters even trolled them by holding up the Tottenham badge between the divide that separates the two sets of fans.
A video posted on social media showed the home end showing the Spurs badge, and was met by an obscene gesture from an Arsenal supporter.
It was a worrying night in more ways than one for Mikel Arteta's side as they suffered a 2-1 loss.
Gabriel Jesus had put them in front, but Adrien Thomasson levelled following a David Raya mistake.
Lens would go on to win 2-1 courtesy of a second-half strike from Elye Wahi.
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Perhaps even more concerning was an injury to Bukayo Saka, with the star man hobbling out of the game.
Arsenal face treble winner Man City on Sunday in the Premier League and could be without the forward.
Asked if he regretted picking Saka following his issues against both Tottenham and Bournemouth, Arteta replied: "No. It was a knock that he had the other day and he was perfectly fine. It was a back-heel, an action that can produce that kind of injury.
"Let's see what the extent of it is and afterwards it's too late. The last few were more knocks than anything else.
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"He hasn't really missed games. We gave him a break against Brentford (in the Carabao Cup) last week and that was all.
"He tried to backheel a ball in the first half and felt something muscular. He felt uncomfortable to carry on so we had to take him off.
"We don't know anything more. It was big enough not to allow him to continue to play the game and that's a worry for us.
"He was really looking forward to playing like every player. It was a big Champions League night. I painted a picture and the type of scenario we were going to face today and they all knew about it.
"But this Champions League is so difficult to win away from home. Today we take a big lesson."