The two Blues stars have been linked with exits from the club this summer.
The two players are two of the most highly rated young players in Europe at present, with Lukaku in particular having shone this season.
The striker has enjoyed his best goalscoring campaign to date and has already talked of how he wants to play Champions League football next year.
That has sparked numerous rumours of top European clubs courting his signature in the summer, while his teammate has similarly been linked with a move away.
Losing both would be a big blow to the club, but Martinez insists that they could cope should they leave.
“If these players were out of contract, it would make you very, very vulnerable,” the Daily Mail reports him saying.
“But if they are under contract, you are in control. These players have an incredible value in the market and it's very easy to replace a player with that sort of value.”
“I'm looking at the valuation of our squad now and it's not something to be fearful of if we lose a player.”
While Martinez may be insistent that Everton could easily replace any players who may leave in future, in reality that is arguably not the case.
The club may have better spending power this summer courtesy of the investment from Farhad Moshiri, but their poor form this season will not have done anything to convince players that Goodison Park is a suitable destination.
In fact, playing with the likes of Lukaku and Stones are arguably two of their main drawing points for prospective signings and keeping them is surely of the highest priority,