ENGLAND SQUAD TO PLAY CROATIA AND SPAIN: J Pickford (Everton), M Bettinelli (Fulham), J Butland (Stoke), A McCarthy (Southampton); T Alexander-Arnold (Liverpool), J Gomez (Liverpool), H Maguire (Leicester), D Rose (Tottenham), L Shaw (Man Utd), J Stones (Man City), J Tarkowski (Burnley), K Trippier (Tottenham), K Walker (Man City); N Chalobah (Watford), R Barkley (Chelsea), E Dier (Tottenham), J Henderson (Liverpool), J Maddison (Leicester), M Mount (Derby), J Sancho (B Dortmund), H Winks (Tottenham); H Kane (Tottenham), M Rashford (Man Utd), R Sterling (Man City), D Welbeck (Arsenal).
Maddison has three goals and two assists since making his £24million move from Norwich to Leicester, while Mount has netted three times for Derby.
They are not the only debutants, with midfielder Mason Mount also called up.
Sancho, meanwhile, becomes the first player born in the 21st century to be named in an England squad.
Ross Barkley and Nathaniel Chalobah are also called up.
Borussia Dortmund's Sancho has risen from the under-19s - Southgate, however, said he would have been selected regardless of injuries - while Mount and Maddison had been part of Boothroyd's under-21 squad.
Injury blow for Leeds United as Barry Douglas ruled out of Brentford clash
Dele Alli, Fabian Delph, Jesse Lingard and Adam Lallana all miss the two Nations League fixtures through injury.
"I think he is progressing really well, he is getting great experience with some game-time at his club and working with outstanding players in training every day which will help him develop".
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). He has been fearless enough to go and play overseas and has started two Champions League games for Dortmund and has also had a big impact from the bench in their league games.
Southgate's side then travel to Seville to take on Spain, who won 2-1 at Wembley in England's first Nations League match last month.
Mason's situation is slightly different.
"To have a player starting in the Champions League is of importance to us", Southgate said. We're always conscious of the balance in bringing young players in but we think they are performing well with their clubs and they are confident players.
Duke and Duchess of Sussex visited Chichester, Bognor, Brighton and Peacehaven
They were full of smiles as they interacted with crowds of people awaiting their arrival at Chichester in West Sussex. The duke and the US-born former actress saw a rare copy of the American Declaration of Independence at Edes House.