Watford boss: Middlesbrough's Johnson destined for England
He said: "Adam needs to be playing regularly and needs a good couple of seasons under his belt.
"Stewart Downing is a terrific player but Adam has something a bit different.
"He is here because Stewart is in the Middlesbrough team, that is the only reason he is not in their side. He has a lot of talent."
The physical nature of Championship football means Johnson is being forced to grow up quickly and Boothroyd believes that can only benefit him.
He said: "Like a lot of young lads who come through the academy system, Adam needs the next level of education - which is playing men's football.
"He is learning what it is to be battered about. He was whacked a few times in our 1-1 draw at QPR, expected to get free-kicks and didn't.
"That is part of growing up. He is a terrific character to work with and he wants to get better. He's still growing so he will be a big guy.
"The Championship is a rough league but he is dealing with it."