The 18-year-old winger is enjoying a sensational start to 2018/19 and has been rewarded with a first Three Lions call-up.
"He's dealing with that and excelling", the England manager said.
"We've got a few injuries in the midfield in particular - Dele (Alli), Jesse (Lingard) and Ruben (Loftus-Cheek) have been missing - and Fabian Delph, so there is an opportunity there".
"You never know with young players what they are capable of until they have an opportunity", the manager told a news conference at the national St George's Park training center. "It's a good time in their development to train with us and see how they adapt".
Sancho signed a new contract with Borussia Dortmund earlier this week and now he has earned his first call up to England's senior team.
"I must acknowledge the hard work and commitment of my staff and the players over the last two years", said Southgate.
"[Sancho and Mount] are both in their formative stages but we think they can play in the group". Sancho has more assists than any other player in Europe, appearing mostly as an impact sub for the Bundesliga giants. He recently signed a new contract with the German club.
Southgate has made a strong impact since taking over from the sacked Sam Allardyce in 2016, evidenced by England's performances at Russian Federation 2018, when the Three Lions reached its first World Cup semi-final since 1990.
The 21-year-old has scored three Premier League goals and assisted a further two since his summer move from Norwich City. "He's started two Champions League games for Dortmund and he's had a really big impact from the bench in their league matches so that's been a really fearless decision from him to go and seek his opportunity".
England kicked-off their UEFA Nation League campaign to a bad start as Southgate's men lost at home to Spain.
There are six uncapped players in total in boss Gareth Southgate's 25-man squad, which has an average age of 23.9.
"We'd have no hesitation in playing them in the games", Southgate said of Mount and Sancho.
Harry Winks, capped once, has returned to the senior team the day after starting for Tottenham against Barcelona in the Champions League, while uncapped Watford midfielder Nathaniel Chalobah is back in the squad after recovering from a long-term knee injury.
Southgate, who became England coach in November 2016, is contracted to stay in charge of the national team through to the 2022 World Cup in Qatar.