If an opposing player held back a counter attack like that, we'd be demanding a yellow card. It's unfortunate, but he was in trouble when he got the first card for time wasting, he couldn't take that chance.
I wonder if this means that Gabriel will actually start the next match. I haven't been happy with this weird makeshift back 4 anyhow.
Exactly, it was a counter and he was baited by the turn. It isn't enough for a yellow imo but it is still a foul. Whether or not it is a yellow is up to the refs discretion. Still you shouldn't be baited with a swivel like that at this level.
You have to be aware you're on a yellow card too. Even if you get baited, you have to rely on your other defenders. He was in Palace's half at the time, so there was time to get back.
I was always surprised that Wenger never seemed to try to bring in one of the Ayew brothers. They seem very much like Wenger players. Smart and technical while not the traditional blueprint of a striker / winger.