The striker thought he had doubled the Reds’ advantage at Anfield after firing past David de Gea on Sunday.
However, Paul Tierney, the video assistant referee, chalked off the goal for a foul on De Gea by Virgil van Dijk in the build-up.
Liverpool went on to extend their lead at the top of the table to 16 points with a 2-0 win, but the disallowed goal was a big talking point.
Walton told The Times: “Manchester United can count themselves incredibly fortunate that VAR chalked off Roberto Firmino’s goal for Liverpool after ruling that Virgil van Dijk had impeded goalkeeper David de Gea.
“I do not think there was enough evidence to disallow the goal.
“Last month, VAR allowed a goal for Everton against United to stand after Dominic Calvert-Lewin had made contact with De Gea at a corner.
“There was more contact in Calvert-Lewin’s challenge than Van Dijk’s but it was not deemed serious enough for the goal to be disallowed.
“Interestingly, [referee Craig] Pawson was the video assistant who allowed the Everton goal at Old Trafford.
“With incidents like these there is a degree of subjectivity and the on-field referee should be encouraged to look at the monitor and make the final decision.
“That way, the viewing public would accept the outcome more easily.”
Keane felt the disallowed goal proved ‘the game’s gone mad’.
He added: “Why he’s disallowed that goal is beyond me.
“How they see that as a foul is beyond me. I can’t get my head around it. He’s got his eye on the ball.