"Typical Arsenal," Merson said on The Debate. "They were cruising along and they were playing well, creating good chances.
"You just think keep it tight, win 1-0 and we've got a chance. Then the goal they give away is just typical Arsenal. I've seen it before.
"It's weak the way they let the goals in. There's no fight, there's no desire at the back. If you are a defender, keeping a clean sheet is the equivalent of a centre-forward getting a hat-trick. It's the same."
The Europa League is Arsenal’s only remaining chance of silverware this season.
It will be manager Arsene Wenger’s final campaign in charge after he announced last week that he will step down in the summer.
Arsenal make the trip to the Spanish capital next Thursday for the second leg (8.05pm).
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