Rashford scored in England's defeat to Spain.
Southgate told reporters: “The only way for him to improve is by playing. He knows we have real belief in him.
“I think we also have to remember his age and what he's achieved in a short period of time.
“He's got a tremendous mentality, he's got great talent, we think he's got everything to go on and be a top player and I'm sure he'll do that."