Replays of the goal suggest it maybe should have been ruled out.
United were 1-0 down midway through the first half after a blunder by David De Gea.
But they responded brilliantly with a goal themselves two minutes later.
Anthony Marital buried his chance from inside the area after Bernd Leno had originally saved a Marco Rojo free-kick.
But it appears as though Ander Herrera evaded the linesman’s flag to set up Martial’s goal.
As the TV replays suggest, Herrera broke away from the defensive line before Rojo took the free-kick.
The offside was marginal but commentator Robbie Savage reckons the flag should have been raised or VAR used.
“It’s a free kick from Rojo, who clips it over the wall, the keeper saves it but you have got to push it out of the danger zone,” Savage said.
“I think Herrera was offside from the initial free kick, he cuts the ball back across for Martial to tap in.
“We will have to have another look at it, but it doesn’t matter the goal has been given.”
Savage then added: “Herrera is just leaning offside as the ball is played.
“VAR would have ruled that goal out as his head and torso was offside.”
At the break Savage added: “1-1 at half-time. I would say that is a fair reflection, it has not been great quality in midfield for United.
“Do they deserve the equaliser? Possibly not.
“The 3-4-3 formation suits Arsenal better, it looks new to United. I think the happier manager at half-time will be Jose Mourinho.”