Boothroyd said: "Darius brings a fear factor to our games now.
"Defenders in this division are scared of him and rightly so, because he scores goals.
"He couldn't buy a goal last year in the Premier League, but that was a great experience for him.
"Now, with the emergence of Tamas Priskin and the arrival of Nathan Ellington, he has had to raise his game.
"He's a lot more alert now and he causes havoc physically."