Lukaku has consistently scored Premier League goals over the last four seasons, netting 15 in 2013/14, 10 in 2014/15, 18 in 2015/16, and a career-best 25 last season.
Rashford believes the timing of the 24-year-old's transfer from Everton to United has come at just the right moment, with the striker firing on all cylinders.
"He's gone through a phase in his career now and he's coming up to his prime years and he has been scoring a lot of goals for the last three or four years," Rashford said, according to Sky Sports.
"I think he is in a good place in his career and he has probably come here at the right time."