The striker, 25, excelled for Belgium in Russia but said he bulked up during the tournament and returned to Carrington overweight.
But former Red Devils right-back Neville struggled to comprehend how it happened with a huge support team.
Speaking on Monday Night Football last night, Nev said: "I struggle with the weight thing and maybe that's just because of how I was as a footballer.
"You could never take a chance with preparation.
"You could play badly, you could give goals away, you could make mistakes, you could miss chances, whatever type of player you are, these things happen in games.
"But actually, don't take a chance with preparation.
"Do I turn up every morning? Do I eat the right things? Do I go into the gym at the right times? Do I follow the programme?
"You can argue whether he has got the right programme and then you start to question your coaching staff and things like that, but I really do question that.
"In the modern age when you have 55 staff supporting every team, nutritionists, strength and conditioning coaches, I struggle to understand how a player can be allowed to feel like he has too much muscle or is overweight. Particularly when he's fit."
Lukaku ended his 997-minute Old Trafford goal drought during the 4-1 win over Fulham on Saturday.
He admitted he put on some muscle, something that happened four years ago after the World Cup in Brazil.
The Belgian striker said: "It was at the World Cup. I just felt great and I think I played great over there, and then when I came back it is a different type of game.
"In the Premier League, I cannot play with the same amount of muscle as international football.
"That was something that when I came back I knew straightaway ‘Nah, nah, I cannot play in this style like this’.
"I had to lose muscle. So you just stay out of the gym, drink a lot of water, and eat a lot of veg and fish and it helps."
Neville also spoke in depth about Raheem Sterling and the racist abuse he received at Chelsea this weekend.
The pair worked together when Neville was an assistant to Roy Hodgson with England and the winger confided in his coach when criticised during Euro 2016.
Neville added: "He came to see me one-on-one in 2016, three or four days before the Iceland game.
"Pre-tournament at Euro 2016 he was getting battered. He was getting so much stick and we were aware of that. The fans were on to him and the media were as well.
"It continued on into the tournament and in the stadiums there were groans and boos.
"He was asking ‘Why is this happening? Why is it so personal?’. He accepted that he would get criticised for England if his performance levels were not high enough but it was so vicious and felt so targeted.
"He didn’t know what to do.
"I was trying to patch him up, to get him to a certain point – and I was not dealing with the underlying issues and probably wouldn’t be able to.
"But now, actually reflecting, I was brushing it aside.
"I kept saying ‘You’re strong, you’re good enough, it’s happened to top players before’, but deep down I understood that there were tonal differences to attacks on him compared to others. "
Neville compared the criticism to that of "blue-eyed boy" Harry Kane at the European Championships.
He said: "For Raheem it was more personal reasons and the language being used was different.
"He couldn’t understand it, he was asking why it was happening.
"We then got knocked out but the abuse he received from fans and the media beyond that tournament was like I've never seen before.
"I've lived closely with Becks, Rooney and at Euro 96 with Gazza, but the nastiness is really there with Raheem."
Manchester United fly to Spain today as they prepare to take on Valencia in their final Champions League group game.
Jose Mourinho's men have already qualified for the last 16 and is expected to field a weakened side with a trip to Liverpool on Sunday afternoon – their 13th league game on TV this season.
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