A deal worth a reported £75million saw Lukaku swap Goodison Park for Old Trafford and the powerful hitman has already shown signs, during pre-season, that he will perform for his new club.
Rafael, now with Lyon, was a vital cog in United's defence during the seven-year period he spent at the club.
The Brazillian won three Premier League titles under now departed coach Alex Ferguson and recalls coming up against Lukaku during the Scotsman's final game in charge.
Turning out for West Brom at the time, Lukaku spoilt Ferguson's farewell game as he netted a hat-trick in a 5-5 draw at the end of the 2012/13 season.
"I've seen Lukaku play at Everton and a bit at Chelsea," Rafael told Omnisport. "I saw him also when we were champions, and he was in West Brom, [which was] maybe my best time in Manchester.
"For me, he is a good player and is at a good age, an age he can evolve, but if he keeps what he did last year and in the last two or three years, he will be an excellent signing."