He missed out on World Cup selection last year, but Boothroyd says Young's move to United is set to ignite his international career.
"He has settled in very quickly - I think he needed another challenge and he has certainly gone to the best club and is playing under the best manager to get that out of him," said Boothroyd, who worked with Young at Watford. "The players of the quality United have will really bring the best out of him.
"Because of the talent that he had and because of the money that we didn't have at Watford, we couldn't go out and buy anybody, so he got his chance.
"I've always been a big believer in giving young talent a chance, although there were times when he played for us that he was decidedly average.
"But he's fiercely ambitious, wants to win things and the fact that he's now playing for United will suit him because you can't get into a comfort zone.
"He'll get Champions League football and add that to his bow - which makes an England manager more ready to pick him."