Boothroyd said: "Because of the injuries the team has got it just gives them an opportunity".
Maddison has three goals and two assists since making his £24million move from Norwich to Leicester, while Mount has netted three times for Derby.
The new contract is understood to include a significant pay rise, which will mean Southgate earns 50 per cent more than he did for guiding England to the semi-finals of the 2018 World Cup.
"It aligns with what we have done in the last 18 months", said Southgate.
"We are always conscious of the balance on how quickly we put those players in, but we think they are performing well with their clubs, they're confident players and we get an understanding of how they and we work".
"We think there are some exciting young players we have been tracking, and it's a great opportunity for us to look at them".
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Mount has been directly involved in 17 goals in his past 18 league appearances across the Dutch Eredivisie and the Championship, including play-off games (10 goals and seven assists).
"To have a player starting in the Champions League is of importance to us", Southgate said.
One of England's weak points in recent years, including at the World Cup, was a lack of creativity and vision in midfield.
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).
"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".
In total, there are six new players in Southgate's latest squad. If he sees out his deal, he will be longest-serving England coach since Bobby Robson (1982-90).
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I feel I still have a lot of football left in me but it is now up to the club. So I see less of my daughters, that's why I prefer morning training.