Maguire's header in extra-time saw United defeat Lyon 5-4 and reach the Europa League semifinals 7-6 on aggregate.
Souness wrote for the Daily Mail: "It was good to see Harry Maguire, one of the game's good guys, become a hero again for Manchester United this week.
"It has been a difficult period in his football life with previous managers looking to show him the exit door and the continuous criticism he has had to put up with on a weekly basis.
"That won't have been easy, but he has shown real mental toughness in dealing with it and continued to turn up and say: 'No, I want to be part of this.'
"I met him briefly in Qatar during the 2022 World Cup and found him to be a thoroughly decent man. He is one of the game's good guys, so fair play to him, it's good to see him getting the credit. I'm pleased he is the fans' hero for once.
Maguire at heart of United fighting spirit
"Maguire was at the heart of a United side who showed they have a fighting spirit on Thursday night, the trouble is, they have only shown it occasionally, and occasionally at Manchester United is no good.
"And as for the goalkeeper Andre Onana winding up the opposition, that's not a good idea. Going by his performances, he's best keeping his head down as football has a habit of coming back to bite you on the backside.
"It's that old chestnut, that for every team who plays Manchester United it is their cup final. The history and magnitude of Old Trafford oozes out of every column of the place.
"You always have to be on it 100 per cent in every game to be a United player, as the opposition always will be. That's the challenge. But this lot haven't been able to deal with that."