The Toon boss thought Nasri should have been sent off for tripping substitute Joey Barton.
Keegan confronted Nasri at the final whistle with Gunners skipper Gallas going to his team-mate's defence.
Keegan said, "I saw Nasri slice down Joey and that's what I was so upset about.
"If it had been the other way round I'm sure Joey would have been sent off.
"Joey came on and got a very good tackle in on Nasri. Then he tripped Joey when he was running back.
"I like Nasri and we talked about bringing him to Newcastle, but he can't do that.
"The referee didn't see it but the linesman did. Under the rules it's a red card.
"I spoke to Nasri at the end and told him I thought it was wrong - he is too good a player to do that.
"He said something back to me but French is one of the many languages I don't speak."