Lukaku fell out of favour at Old Trafford under Ole Gunnar Solskjaer in the second half of the 2018-2019 campaign with Marcus Rashford being preferred.
Reported Inter target Lukaku has acknowledged he would be interested in a move to Italy and hinted last week he may be on his way out of Old Trafford.
The 26-year-old scored 15 goals in 45 appearances for United in all competitions last term and Martinez stressed the importance of Belgium's leading man finding a new club.
"It is clear that he has to leave Manchester United," Martinez said.
"It would be a good thing for both parties. It is now important that Romelu finds the right club."
Belgium's record scorer Lukaku, reportedly valued at £80million by United, scored three goals for Martinez's men across two Euro 2020 qualifiers earlier this month.