The 18-year-old Manchester United striker helped the Young Lions to a 6-1 win over Norway in Euro2017 qualifying in Colchester on Tuesday, prompting words of adulation from manager Southgate.
"His mentality all week – he came with great humility," said Southgate of Rashford who went to Euro2016 with the England senior team.
"He's young, so you hope that's going to be the case, but it isn't always the case.
"It's a complex journey, if you've been up to the senior team, coming down. It's an environment that people can settle in quickly.
"I think he's enjoyed the week and the work with the rest of the team – they've made him feel welcome and he's responded to that.
"I'm delighted for him, but there were also lots of other very good and important performances."