Maguire came on at haftime during England's 3-1 friendly win over Scotland on Tuesday night.
His arrival was greeted by jeers and boos and the mocking was compounded when he scored an own-goal to give Scotland hope of a way back into the game.
After the game, Southgate spoke about his annoyance at the sustained criticism Maguire had been subjected to in recent years.
Southgate defends Maguire
Speaking to Sky Sports News, the England boss said:
"He's just come out and spoken to the media, which I think shows the resilience and the character of him.
"We're now in a situation where, because of people in our own country, the opposition think they can have fun with him. What's that about?
"He's been an incredible player for England in one of the most successful teams we've had - critical for what we've been doing.
"We allow him to be open to that? It's a joke; an absolute joke. It makes me livid.