The towering striker joined Everton permanently from Chelsea in July 2014 for a fee of £30m and has kicked on to become one of the Premier League's most reliable players in front of goal.
Leading the way in England's top flight with 21 goals and six assists this season, Lukaku has been linked with a return to London to lead the line for Antonio Conte's high-flying Blues.
At just 23 years old, the future is certainly bright for Lukaku at whatever club he turns out for, but his national boss, and former Everton manager, Martinez, believes Chelsea – or any other team, for that matter – will have to fork out an inordinate amount of money to land their man.
“It’s difficult to say [if Everton can keep hold of Lukaku],” Martinez, who managed Lukaku for three years at Goodison Park, told FourFourTwo .
“What’s important is that as a club, if you lose a player, you lose him on your own terms. Rom has doubled or trebled the initial fee that Everton paid for him."
Harking back to when the Toffees were originally trying to sign the striker, Martinez admitted that the rules of the club's long-standing financial structure were broken in order to take a wise gamble on a hot prospect.
“In my eyes, potentially, he was worth more money. We knew he’d become the player he is now,” the Spaniard went on.
“Victor Moses became a really special player with Wigan, Ashley Williams became captain at Swansea, James McCarthy and Gareth Barry were really important for me at Everton.
“But Romelu was different – you were breaking all of the established rules to bring him to the club.”