Scanlon calmly found the bottom corner of the net, with an assured finish, after being teed up by Aaron Drinan.
After the game, Robins boss Ian Holloway told reporters: "Am I surprised? No, I am not. Am I pleased? Yes, I am, because I knew he would finish that. That is what he does.
"He's delighted that Man United let us have him. It's what he can do, he plays for his national team, and is only 19. He shows all sort of maturity, he can play two or three different positions."