Duffy, who played alongside the Belgian international at Everton, described the forward as "one of the best strikers in the world".
"The goals he has got show what he can do. We were both at Everton at the same time. He's done brilliantly and is now regarded as one of the top strikers in the world," he told The Express.
"He's still young. He has a lot of pressure on him trying to deliver goals for Manchester United – it's not an easy task."
Lukaku has hit 24 goals during his debut season with United.