The Manchester United striker fired home a hat-trick on debut for the U21 side in their 6-1 win over Norway in Colchester on Tuesday.
Rashford had dropped down to the Young Lions despite representing the senior team at Euro2016 and after such a fine performance, Southgate quipped that perhaps the 18-year-old could be off to Sam Alalrdyce's Three Lions in the future.
AfterRashford was subbed off late in the game, Southgate revealed: "I just said, 'You've enjoyed your week haven't you?'
"I was tempted to say, 'It was nice working with you!'"
Rashford will now return to Manchester to prepare for Saturday's Manchester derby against Man City at Old Trafford.