Winger Melligan serves the second game of his three-match ban after being sent off at Norwich last week, while skipper Purches is still sidelined as he recovers from a fractured cheekbone.
Andrew Cave-Brown will keep his place at right-back in Purches' absence, with the experienced defender hoping to resume training this week with the aid of a protective face mask.
Midfielders Adam Chambers (calf) and Sean Thornton (knee) are still on the sidelines.
Adrian Patulea, Loick Pires, Luke Ashworth and James Scowcroft are pushing for starts should boss Geraint Williams opt to shuffle his pack for the game.
Kick-off at Brisbane Road is at 7pm.