Rashford was one of few bright sparks for Southgate, scoring both England's goals in a 2-1 defeat to Spain and 1-0 victory over Switzerland in the past week.
However, he has struggled to hold down a regular first-team place under Jose Mourinho at Old Trafford and will be suspended for United's next three domestic games after being sent off at Burnley before the international break.
Southgate, though, used the example of England captain Harry Kane and Cristiano Ronaldo to show that at just 20, Rashford has plenty of time on his side.
"We have to remember that Harry, at that age, had hardly played anywhere near the same number of matches for Spurs," said Southgate.
"Even with some of the step-overs and things Ronaldo was doing at that age he wasn't converting them into the number of goals.
"So we have to give him time to develop. He has still played a lot of football, although maybe not continuous starts, and I'm really pleased for him that he leaves us full of confidence."
Southgate has raised concerns over the lack of opportunities afforded to English players in the Premier League, making the pool he has to select from ever shallower.
Yet, he refused to criticise any of the top six managers, all of whom are foreign, for putting their own interests first.
"It's not my business to interfere with club managers. They have a difficult job to do and they have big squads at the top six, with huge competition for places.
"Jose thinks the world of Marcus. He's a huge admirer of him, but he has got his job to do. There is huge competition for places at his club so you can understand that."
Ruben Loftus-Cheek is another of England's squad that advanced to the semi-finals of the World Cup struggling for minutes at Chelsea.
The midfielder made his first start of the season against Switzerland having previously just come off the bench twice for Maurizio Sarri's men.
"Right now I am at Chelsea and my focus is at Chelsea and learning at Chelsea," said Loftus-Cheek. "That is in the future and we will deal with that when it comes.
"I'm not sure when I am going to play. It is just down to me working hard in training."