It is clear Southgate sees the youngster, who was on the mark against West Ham last Saturday, as a potent weapon as England begin qualification for World Cup 2022 and also head to next summer's Euros.
Southgate said: “What you saw with Mason and his finish at the weekend is his biggest strength for me.
“One chance, really, and a fantastic touch to put him in position and a brilliant finish.
“That is what we were seeing towards the end of the season and why we called him into the squad for September."
Southgate explained: “He has got great support at his football club. He has got people that care for him there and will bring him into the team and out of the team at the right times.
“Hopefully, he will get a run of games now and at a club like Manchester United, every game is important, so he will always have to perform under pressure and under high expectations.
“It is great for their young players that they already have to deal with everything they have to deal with for England."