Boothroyd, manager of Watford from March 2005, when he took Young and his team-mates into the Premier League, reckons United have bought themselves a hybrid of Nani and Ji-Sung Park, a versatile player who can play on the wing or upfront, and will end up being Ryan Giggs' long-term successor.
He told the Mirror: "Sir Alex will have done his homework and won't be taking a player he didn't think could add to them.
"Ashley has the work-ethic Park has got and I think he has got some of the quality and unpredictability to beat a player and create like Nani and Luis Antonio Valencia have got.
"He is sort of a mixture or hybrid of them - plus he is British which makes a big difference.
"He has done very well since he left Watford, he has had a little taste of European football, he has had a little taste of international football and now he is going to go on the biggest stage of all.
"It is a real competitive environment he is going into but it is one I am sure he will relish and will do really well in.
"The most important thing you have to do at United after you have won is win again.
"That will suit Ashley as he likes to win games and is a winner."