The midfielder underwent a MRI scan on Monday to discover the extent of his ankle injury. He will be out for at least a week.
Joe Mason could start.
Gunnarsson was one of nine summer signings by Mackay after 12 players left following Cardiff's failed play-off campaign last season.
Brighton striker Will Buckley has been ruled out of the trip after suffering a hamstring injury at Portsmouth on Saturday.
Craig Noone replaced Buckley at Fratton Park and could be handed his first league start of the season, although fellow wingers Kazenga LuaLua (hamstring) and Will Hoskins (hip) may be back in contention.
Defenders Adam El-Abd (foot) and Tommy Elphick (Achilles) are set to return to training after recovering from the injuries they suffered at the end of last season, but they remain a couple of weeks away from potential first-team starts.