You're right it's a very different situation when he was at Real Madrid, but it's the same situation when he was at Manchester United in the past. It's not about comparing the performance of one player to another. But here the role of the players in their position will determine the team's performance.
Maguire, yes this one player is still very questionable his role for Manchester United, whether he really plays for Manchester United or plays for the opponent. Lol
Seriously, how can anyone think Ronaldo doesn't perform good on Manchester United? he's on top 3 scorers and shoots, while his club is struggle with internal problem and have a bad record. This mean Ronaldo already carrying Manchester United to become better, but as we know football as a team sport, can't only relying single player.
Looking how Maguire play style on Manchester United, I'd think he's a Central Attacking Midfielder rather than a Central Back.