Much anticipation surrounded the potential promotion of teenagers Sancho and Mount following their fine starts to the season at Borussia Dortmund and Derby County respectively.
"We've got a few injuries in the midfield in particular - Dele (Alli), Jesse (Lingard) and Ruben (Loftus-Cheek) have been missing - and Fabian Delph, so there is an opportunity there. They're confident players, we'd like to work with them now so that they get an understanding of how we work and we get a closer look at them".
"He's a player that we know has good technical quality, is grasping the tactical side of the game - I was interested to read his interviews this morning on that - so it's a good opportunity for him to come in".
After minimum game time with the senior team, Sancho took the bold move to go to German Bundesliga side Borussia Dortmund youth team in 2017.
"I feel as if there's still a chance to improve what we're doing", Southgate said after extending his contract.
"They're both still at formative stages but we think they can have an impact with the group and we'd have no hesitation in playing them in the games".More news: Key senators undecided as U.S. Senate poised to vote on Kavanaugh
Sancho's five Bundesliga assists are more than any other player in Europe's big five leagues this season, and he has set up a team-mate once every 25 minutes on average in the Bundesliga.
Football Association chief executive Martin Glenn had earlier insisted England "will win a World Cup, we are going to win a Euros".
Maddison has three goals and two assists since making his £24million move from Norwich to Leicester, while Mount has netted three times for Derby. "He's started two Champions League games for Dortmund and he's had a really big impact from the bench in their league matches so that's been a really fearless decision from him to go and seek his opportunity".
They are not the only debutants, with midfielder Mason Mount also called up.
England lost to Spain 2-1 in its first match in the newly formed Nations League.
Harry Winks, capped once, has returned to the senior team the day after starting for Tottenham against Barcelona in the Champions League, while uncapped Watford midfielder Nathaniel Chalobah is back in the squad after recovering from a long-term knee injury. If he sees out his deal, he will be longest-serving England coach since Bobby Robson (1982-90).