"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 global career a year ago.
Boothroyd said: "Because of the injuries the team has got it just gives them an opportunity".
"This is a great opportunity for us to look at them".
England squad: Bettinelli (Fulham), Butland (Stoke City), McCarthy (Southampton), Pickford (Everton); (Liverpool), Gomez (Liverpool), Maguire (Leicester), Rose (Tottenham), Shaw (Man Utd), Stones (Man City), Tarkowski (Burnley), Trippier (Tottenham), Walker (Man City); Barkley (Chelsea), Chalobah (Watford), Dier (Tottenham), Henderson (Liverpool), Mount (Chelsea), Winks (Tottenham); Kane (Tottenham), Maddison (Leicester), Rashford (Man Utd), Sancho (Borussia Dortmund), Sterling (Man City), Welbeck (Arsenal).
Maddison, 21, who has won four England Under-21 caps, was watched by Southgate when he helped the Foxes beat Newcastle United on Saturday.
The youngster is now leading the charts for the number of assists (five) this season across Europe's top five leagues, managing to provide a goal for a teammate for every 25 minutes of action.
As the global break is upon us, England manager Gareth Southgate has named his latest squad for the Nations League games. He's had two starts in the Champions League this year, he's a player that is explosive, is really creative in the final third, has had a big impact in terms of assists in the matches that he's played, can score goals.
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Sancho joined the "BVB" in the summer 2017 and has made 18 league appearances, providing two goals and nine assists so far.
Jadon's is an unusual story.
"He's been courageous enough to go overseas and play", Southgate said.
With England, Sancho has won Federation Internationale de Football Association U-17 World Cup in 2017 the UEFA European Under-17 Championship Golden Player and was part of the Team of the Tournament. This year in the Championship he's created more chances than any other player in the league.
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2017 Federation Internationale de Football Association U-17 World Cup but only played in the group stage before he was withdrawn out of the tournament by Borussia Dortmund.
In the 25-man squad, 23 were born in the 1990s.
In total, there are six new players in Southgate's latest squad.