Another attacking midfielder, 21-year-old playmaker James Maddison, also received a first call-up after impressing in his first season in the Premier League at Leicester.
"In a different era it would take 100 appearances in the top flight before they got a cap but we live in different times".
"He's a player that is explosive, is really creative in the final third, and has had a big impact in terms of assists in the matches he has played, can score goals and has had a big impact in our junior teams in the last couple of years".
"Quite a few of them have trained with us at various times here anyway so we're excited to see them".
Both Southgate and his assistant Steve Holland also signed new four-year deals on Thursday, keeping them in charge until 2022.
Injuries have led Southgate to make changes to World Cup semi-finalists England's squad.
"I think very often we hold youngsters back too much".
"At Dortmund he is required to do a job defensively for the team and he does it, so he's an exciting player we're looking forward to working with". Two of them have returned to the group after injury: Watford's Nathaniel Chalobah and Tottenham's Harry Winks.
"Well, he's had a long period injured at Chelsea a year ago, 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. Having led the team to fourth place in Russian Federation this summer, Southgate - whose salary is believed to have risen to £3million a year - revealed he had no doubts over wanting to stay.
We'd have no hesitation on playing either of them in the games coming up.
"Then they have got to fight for their place in the U21s squad then because that's not bad either".
"Where actually if you don't have a target like that... we started to work back from there".
Discussing his extension, Southgate said: "You can have great relationships with your clubs and I did with mine - I had really special relationships with all three of my clubs - but emotionally the national team has always been the biggest thing for me".
Maddison, 21, has also been rewarded for a series of influential displays with Leicester City, although Daniel Sturridge has been overlooked despite finding form for Liverpool.
"From Joe's perspective, we don't need to see Joe in a game at this stage to know what he can do". The move to Burnley has obviously worked for him - he looks confident in his game.