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.
Boothroyd said: "Because of the injuries the team has got it just gives them an opportunity".
"I think this aligns with what we've done in the last 18 months, really", said Southgate, who helped bring about the end of Wayne Rooney's worldwide career past year.More news: UFC 229 Khabib Vs McGregor Vlog Episode One
He was part for the U-17 team that won the 2017 Federation Internationale de Football Association U-17 World Cup but only played in the group stage before he was withdrawn out of the tournament by Borussia Dortmund.
Gareth Southgate highlighted the importance of the Football Association's desire to develop youth players in his decision to sign a new four-year deal as England manager.
Several other uncapped players are also involved: Fulham goalkeeper Marcus Bettinelli, Southampton goalkeeper Alex McCarthy and Watford midfielder Nathaniel Chalobah making up the sextet of debutantes.
Mount has been impressive for Championship side Derby, where he is on loan from Chelsea, with the 19-year-old having scored five goals this season.
"Had a big impact with our junior teams over the last couple of years and he's really diligent without the ball".More news: Former Classmates of Brett Kavanaugh Say They've Been Ignored by The FBI
"Well, he's had a long period injured at Chelsea previous year, so that wasn't a reflection on the quality of the player that he hadn't been with us, but he wasn't getting any football, really", Southgate said.
"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".
"I am delighted to have the opportunity to lead the national team through the next two major tournaments", he said.
"Where actually if you don't have a target like that... we started to work back from there". It's really insane right now - we hope we can continue in this form.More news: South Korea jails ex-president Lee Myung-bak