Ahead of Friday's meeting with World Cup finalists Croatia in Rijeka, Southgate said the untested players had done enough in training over the last week to justify their selection.
"It's not experimentation. They are players we believe in," the manager said. "OK, they are relatively inexperienced at the highest level but players we have enjoyed working with and they have shown in training this week they can handle this level."
The England squad also includes uncapped Watford's Nathaniel Chalobah, who has played only four minutes in the Premier League this season, while Tottenham midfielder Harry Winks was recalled after returning from injury.
"We are here to try and get a result so the team we play will be a team we believe in. There is nobody in the squad that is here that we are not thinking of putting into the team," Southgate added.
Croatia beat England 2-1 after extra time in the World Cup semi-finals earlier this year and midfielder Luka Modric had suggested they were underestimated by the English media.
Manager Zlatko Dalic agreed with the Real Madrid midfielder.
"We felt a little underestimated by the English media, not the head coach or the players. There's always a level of respect between the teams," Dalic said.
"You always need to respect your opponent but also have a self-belief in your team and your possibilities."
Dalic said Croatia had a number of injury issues ahead of the match although forwards Andrej Kramaric and Ante Rebic were back after missing games against Portugal and Spain.
England, who are second in Nations League A Group 4, face third-placed Croatia on Friday before meeting leaders Spain in Seville on Monday.
(Reporting by Shrivathsa Sridhar in Bengaluru; editing by Sudipto Ganguly)
