Interim mentor Walmsley on Tuesday informed Simon he had no future at the club beyond this season but admits he'd love to have the injured frontman back on the park before he leaves.
The Mariners host Perth Glory in Gosford on Sunday and Walmsley said if Simon, who is sidelined with a groin injury, was fit he'd be starting.
An eight-season veteran of the club, the hard-running 28-year-old is the Mariners all time leading goal scorer with 41 goals in 148 A-League appearances.
Walmsley admits with Simon ruled out the club has lacked any real attacking firepower.
But with only six games left he's hoping the striker can return before the end of the season to attract another deal.
"Matt's one of the most respected players at the club and as hard as that decision was, it needed to be made and everyone needs to move on," Walmsley said.
"Matt's responsibility is now to get himself fit. I'd love to have him available right now. If he was he'd be in the team for Perth.
"But that's not the case, so the best thing he can do is get himself back on the park and hopefully we can put him in the shop window so he gets the best deal for himself."
With their finals hopes all but over after their loss to Adelaide, the Mariners will be playing for little more than pride on Sunday.
Former front runner Glory meanwhile, has slipped to fourth on the back of an eight-game winless run.
Sunday's game will be its first of three straight matches against the bottom three clubs for Kenny Lowe's side, so it'll be its best chance to turn things around.
Lowe will have Mitch Nichols back from suspension but keeper Danny Vukovic looks set to miss the match with injury.
