Despite a 1-0 loss to Brazil on Saturday to a goal from 17-year-old sensation Endrick, Southgate is desperate to see how his third-ranked team match-up against fourth-ranked Belgium on Tuesday.
"Similar to (Brazil) we're talking about a high-level game, brilliant experience for the players, a chance to see new players again, build towards the summer," said Southgate after the five-time world champions ended his side's 10-match unbeaten run.
"We deliberately took two really high-level games and of course we've had to put out a very different team to the one we might have envisaged.
"So, we've still learned an awful lot ahead of the summer that's really important. You know, some of these boys are pushing for starting positions, some are pushing for places in the squad.
"You need to see them against that sort of level of opposition to see if they're going to be able to excel in the summer."
Harry Maguire, Kyle Walker and Sam Johnstone have all withdrawn from an already depleted England ahead of the game with Belgium at Wembley.
Walker and Maguire started Saturday's match but have now left Southgate's squad, as has backup goalkeeper Johnstone.
"Maguire, Walker and Johnstone have returned to their respective clubs for assessment having sustained injuries in recent days," the FA said in a statement.
Manchester City's Rico Lewis, who made his debut against North Macedonia in November, and uncapped Burnley goalkeeper James Trafford have been moved into the senior squad from the England Under-21 set-up.
England captain Harry Kane missed Saturday's game with an ankle injury and had already been ruled out of facing Belgium.