The Chilean had been booed from kick-off due to his controversial Arsenal exit last year.
Sanchez has only netted four goals for Manchester United since swapping places with Henrikh Mkhitaryan in January.
And Arsenal fans were more than happy to welcome the attacker back with abusive chants and whistles whenever he touched the ball.
But when Sanchez scored in the first half, it was Manchester United fans’ turn to rub salt in the wounds – and they did so in their droves on social media.
“Alexis Sanchez at the Emirates? The game. I love it,” one wrote.
“TEARS IN MY EYES FOR ALEXIS SANCHEZ AFTER GETTING BOO’ED ALL GAME,” one said.
“Alexis Sanchez showing he’s bigger than Arsenal,” another added.
“Sanchezzzzzzzzz!!! Had to be him against Arsenal,” one said.
“The way Arsenal ex players come back to haunt them is amusing. Today it is Alexis Sanchez,” another wrote.
Speaking this week, Ole Gunnar Solskjaer predicted the attacker would relish hostile conditions.
“He’ll be involved,” confirmed Solskjaer.
“I think he’ll love it if the crowd turn against him.
“As a player, pride kicks in and you think ‘I’ll show all of you’.
“He’s been fantastic in training, his attitude has been spot on so hopefully he’ll enjoy the game.”
Unai Emery said: “He [Sanchez] has been starting on the bench after an injury but if he plays we need to work to stop him because he is a very good player.
“Now this [United] team is different and they are playing with more confidence. They lost a lot of points in the season but now they are very dangerous and in a very good moment. They are winning and whether he [Sanchez] plays with one player or another, they are [delivering] big performances.
“He is a very good player. When I was at PSG, we thought we could help him. We spoke about a lot of important players and one was the possibility of Alexis [joining] but it was not possible.”