Johnson, who helped nurture the Stevenage born star, believes he can go on to be a success at Old Trafford.
Johnson told talkSPORT: "He was well looked after, the youth department [at Watford] was bringing in decent players and with Ashley you could see that he had that Premier League athleticism.
"He was only a slight lad and one of the worries with him was going to be strong enough for the top level but certainly as he grew into that man's body then he had the technique and he could drift past people.
"He's a very comfortable runner and with good technique and good ability. He was destined for the top.
"Alex Ferguson doesn't make too many mistakes and now [Young has] got that experience behind him he's probably ready for the top club in the country at this moment in time and I'm sure he will be big at Manchester United. I think he's got the technical ability that the top club in Europe will be looking for.
"Ashley is one of the best in this country at this moment in time, in Europe at this moment in time. Don't forget that Manchester United are the second best team in Europe so you can't get much better than that.