Maguire was initially awarded a penalty in the second half after going down in the West Brom box following contact from Semi Ajayi, though referee Craig Pawson overturned his decision after reviewing the footage on the pitchside monitor.
Offside would have come into play anyway had he stuck with his decision, but Souness nonetheless hit out at Maguire for the way he hit the deck.
“It’s clearly offside and he goes to ground as though he’s been hit with a baseball bat. Diving!” said the Scot, speaking in his role as a pundit for Sky Sports.
“He said he thinks it’s a penalty. It’s not enough to bring him down the way he goes down. He throws himself to the ground. It’s not a penalty.”
Souness added: “This is such a bad result for Manchester United. Maguire has just said it’s a difficult place to go. West Brom have won one game at home this season – against the bottom team. How is that a difficult place to go? That’s a difficult place to go if you’re second or third in the league?
“West Brom had the better chances. That is an exceptionally poor performance and exceptionally poor result for Manchester United, when you think they’ve got a chance of putting the pressure on Manchester City. Nah.”
Maguire felt he deserved a penalty, telling Sky Sports after the game: “I was certain it was a penalty, I don’t understand why he (the referee) has even gone to the monitor. I felt a tug on my shoulder, it was slight, but there was also a clip on my heels and I was confident it was going to be given as a penalty
“But it seems decisions are going against us at the minute, although we can’t rely on VAR decision, we’ve got to do more in the game.”