The Poland international made the gesture before he walked off down the tunnel.
UEFA have yet to confirm whether the keeper will face formal disciplinary charges over his gesture but Gunners boss Arsene Wenger says the player will definitely be warned by his club.
"We will deal with that internally," he said. "We don't agree with that.
"Yes, it (the gesture) was frustration, but we are professionals, me included, and we have always to master reactions. Wojciech is part of that as well."
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