The 26-year-old has struggled to show his best form since joining United in a £90million move from Everton in the summer of 2017.
Lukaku scored 15 goals in 45 appearances for United in all competitions last season.
“It is clear that he has to leave Manchester United,” Martinez told Belgian outlet Het Laatste Nieuws.
“It would be a good thing for both parties. It is now important that Romelu finds the right club.”
Lukaku has confirmed in an recent interview that he would be interested in a move to Italy, as he has been strongly linked with Serie A outfit Inter.