The former Everton striker has scored seven goals in the Premier League since his big-money move to Old Trafford over the summer.
Following his brace, which ended a run of seven matches without a goal at club level, Lukaku spoke confidently about his abilities and remarkable statistics for somebody so young.
However, the frontman was back on target after bagging a brace in Belgium's 3-3 draw with Mexico on Friday night - becoming his country's join-leading goalscorer in the process.
What's it going to take for Manchester United to get back in the Premier League title race and potentially catch table toppers Manchester City?
"If you put Lukaku at the other end, it would be a different outcome", Neville told Sky Sports.
At the end of last month Mourinho said Lukaku was "untouchable in my team" before saying "I have to protect my players when I feel they deserve it".
"When you face a team like us it's hard for opponents".
The Red Devils are set to welcome back Paul Pogba, Zlatan Ibrahimovic and Marcos Rojo in the coming weeks.
"Lately we haven't performed like we did at the start of the season", Lukaku admitted. That's the sort of thing you have to go through in the season, but now there are players coming back from injury and hopefully we can deliver performances even better than at the start of the season.
Lukaku has been an ever-present in United's starting line-up in the league but the looming return of Ibrahimovic - who was an automatic selection by Jose Mourinho last season - means he will face competition to lead the line.