Maguire was injured during the latter part of the 2020/2021 season with Manchester United but he was included in the squad for the Euros by Southgate.
England are favourites to top their group now after their victory against Croatia and Neville believes the Three Lions should start preparing their best center-back for the next round of the competition.
Manchester City defender John Stones and Tyrone Mings were at the heart of defence when England beat the Croats last Sunday but Neville does not believe the duo what it takes to excel should they meet a superior team later in the tournament.
"I think Tyrone Mings did really well - it was an important match for him. Looking at John Stones and Mings as a partnership before the tournament, you would have concerns against the best teams.
"I know people have dismissed Croatia after the game, but not many were dismissing them before the game, so I would still say that's a really good win for England against a good team.
"I think the reason Kieran Trippier played at left-back was his experience and know-how, meaning Mings had experience either aside of him.
"If Harry Maguire can get back to a level, he will come back in. But if he comes back in, I think if it's not against Scotland, it has to be against Czech Republic. You couldn't make Harry Maguire's first game back a knockout game.
"Maguire must surface in the next two matches, otherwise I fear you won't see him in the tournament, and it would be Stones and Mings to continue."
