The three have been linked with moves away from Everton this summer.
There is much speculation surrounding Everton at present, with the majority focused on who will be their next manager.
However, there is also uncertainty surrounding several of their star players have been linked with moves away.
Chief among those is striker Lukaku, whose performances this season have drawn attention from across Europe.
He is not the only exciting talent at Everton garnering attention, and Merson says if they were to lose Lukaku, as well as John Stones and Ross Barkley, they would be a mid-table side.
“I think it’s a good job if you keep those three,” he told Soccer Saturday.
“If you can keep them, then you’ve got a chance. If they all go, then they’re up against it Everton and all of a sudden a bottom half team.”
Everton certainly do face a fight to keep hold of those three players this summer after a disappointing campaign for them.
Lukaku seems the most likely to leave, with his agent appearing to be determined to secure his player a move to a European giant.
The trio could always decide to stay though as they have nothing to lose in giving a year to whatever new manager comes in as the replacement for Roberto Martinez.