Speaking ahead of the two sides' friendly derby in Houston, the City captain was full of praise for his international team-mate.
"Romelu has got one incredible ability," he explained. "It is his desire to score goals. A lot of people have commented on Romelu over the years and he has taken a lot of criticism."
"But his biggest strength is his belief in himself and the ability to always score goals and have that drive constantly."
"I think that is what Manchester United bought and that is what they are going to get from him."
"Therefore they have added a good player to the squad and it is going to increase the competition in the Premier League for the title."
Lukaku joined United for a fee that could rise to £90m as Jose Mourinho's side look to end a four year title drought.