Lukaku, 26, joined United from Everton for £75m in July 2017 and has scored 42 goals in 96 games at Old Trafford.

Belgium's record scorer has been linked with a move this summer after falling down the pecking order under United boss Ole Gunnar Solskjaer.

"It is now important that Romelu finds the right club," Martinez told Belgian newspaper Het Laatste Nieuws.

"It would be a good thing for both parties."

Martinez, who has managed Lukaku at Everton and now at international level, added: "It is clear that he has to leave Manchester United."

Lukaku, who has been heavily linked with a move to Inter Milan, has a contract at United until 2022.

He scored 15 goals in 45 games last season but Marcus Rashford was often favoured in attack by Solskjaer.