On leaving Manchester City for Borussia Dortmund on transfer-deadline day last September, Jadon Sancho declared that "the time is right for a new challenge, where I can to start to fulfil my potential".
Sancho, a gifted wide midfielder who is 17, joined Dortmund from Manchester City past year and has broken through this season, scoring one goal and providing seven assists in nine games mostly off the bench.
Reports suggested he would be handed his first U21 call but an impressive performance and an assist against Monaco on Wednesday changed Southgate's mind.
Here are the things to know about the youngster Jadon Sancho who is set to make his first appearance for the England senior team Three Lions.
The 19-year-old Mount, a member of Chelsea's FA Youth Cup-winning teams of 2016 and '17, spent last season on loan at Dutch club Vitesse Arnhem and is excelling for a Derby side that is bidding for promotion back to the Premier League under Frank Lampard, a former Chelsea player.
Mount, now on loan from Chelsea at Derby, is another to have experienced football overseas, with the 19-year-old having spent last season at Dutch side Vitesse Arnhem.
Ross Barkley and Nathaniel Chalobah are also called up.
Dele Alli, Fabian Delph, Jesse Lingard and Adam Lallana all miss the two Nations League fixtures through injury.
"It aligns with what we have done in the last 18 months", said Southgate. "He's been fearless enough to go overseas and play", added Southgate. He has been courageous 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. 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.
Mason's situation is slightly different.
Jordan Henderson has the most caps (46).
In the 25-man squad, 23 were born in the 1990s.
In nine games, he has scored once and provided eight assists.