Jude Bellingham struck in stoppage time to earn an impressive England a 2-2 draw with Belgium after a Youri Tielemans double had threatened a smash and grab victory in an entertaining friendly.
Manchester United's Mainoo, who made his debut as a substitute in England's 1-0 defeat to Brazil on Saturday, impressed again in a midfield three with Bellingham and Arsenal's Declan Rice with a performance that earned strong praise from his manager.
"He gives us a different profile of midfield player to anything else we’ve got and we’re really pleased with what he's done," Southgate said. "You can’t believe his age really.
"His attributes are there for everybody to see, that ability to receive under pressure and ride challenges, to manipulate the ball in tight areas."
Southgate also singled out West Ham's Jarrod Bowen and Brentford's Ivan Toney for praise, both of whom may have forced their way into England's 23-man squad for the upcoming European Championship in Germany.
Toney, who started up front in captain Harry Kane's absence, scored England's first equaliser, converting from the penalty spot after being brought down by Jan Vertonghen following excellent build-up play from Mainoo and Bellingham.
"At 18 I was probably playing on the Xbox and he is out there being a man and playing for England," Toney said of Mainoo on Sky Sports News. "He just glides with the ball and I'm sure there's more to come from him and he will go right to the top."
Injuries to several first-team regulars have hampered Southgate's squad, but he acknowledged the benefits of being able to offer other players more game time.
"Definitely some players have emerged positively from the opportunities they've had," he said. "So we’ve perhaps got more depth in one respect, but the injuries are a concern."
