Maguire has lost his starting place in the Manchester United team this season and, after some clumsy displays for England, he had to endure a succession of questions about his form at a news conference.
Southgate, however, who has often described him as his rock, said: "It's not for me, it's for us, because he is important for England.
"We wanted to build our team to have the capability to play from the back and Harry and John Stones, who have played the majority of the matches across the last five years, have provided an incredible platform to build on.
"We won't appreciate what stability that's given us in possession until they're not there. And that's the start point, as they also defend well and fight for each other.
"It's great for Harry to get this 50th cap, it's well deserved. He's played a major part of us getting two big tournament performances and we all want this to be the third."
Maguire went off after 70 minutes of the opening 6-2 win over Iran after feeling ill but said he was fully recovered and raring to go.
Well used to dealing with criticism, Maguire was unfazed by that line of questioning. "I have great belief in myself on the training pitch, I give my all, I give my best," he said.
"Nobody likes being criticised but I've played with Cristiano Ronaldo for the last couple of years and he's one of the greatest players ever and gets criticised day in, day out. So if it's going to happen to him I think it's part and parcel of the game.
"What I always tell myself is just go in and do your best and that's all you can do."
Southgate said he had been uncomfortable with English media headlines describing their World Cup group as easy, noting that England had never beaten the United States in a competitive match - infamously losing in the 1950 World Cup and drawing in the 2010 tournament.
"We're good at talking highly of ourselves as a nation on the basis of little real evidence," he said. "So what we've got to do is perform on the field. We know that we'll play a highly motivated team, perhaps even more motivated because of some of those types of headlines, but we've got huge respect for our opponent.
"They are a team that press the ball really well. With respect, they're probably a level of player that has more experience of playing in European leagues (than Iran).
"I think tomorrow will be very different to the first match. We know a lot of their players from our league and we know the quality that they have and the athleticism that they have. So we've got to be at our best."
