The former Manchester United striker, who has scored six goals in 11 games since joining from Sunderland during the January transfer window, is still struggling with the shin injury picked up against Blackburn on Easter Monday.
Mackay is hopeful to have Campbell available for the weekend's visit of Nottingham Forest, but Joe Mason is set to continue in attack against Barnsley after deputising against Watford at the weekend.
Defender Matt Connolly is a major doubt after limping off with a groin strain in the opening 10 minutes of Saturday's goalless draw at Vicarage Road, a result that leaves the Bluebirds needing eight points from their last seven matches to make sure of automatic promotion.
Barnsley manager David Flitcroft is waiting on the fitness of Kelvin Etuhu.
Etuhu, operating in midfield under Flitcroft, picked up a knock in Saturday's goalless draw at Crystal Palace and was replaced just after the hour-mark by winger Jim O'Brien.
Flitcroft, who has no other new injury or suspension worries, could be boosted by the return of defender Martin Cranie (hamstring), who failed a late fitness test ahead of the Palace game.
Stephen Foster was recalled to the Tykes' defence on Saturday and will continue if Cranie fails to make it.
Striker Reuben Noble-Lazarus, defender Jimmy McNulty (both ankle), and Rory Delap (calf) remain sidelined, while left-back Scott Golbourne is out until pre-season due to a knee injury.
The Tykes, who have lost only one of their last five matches, are bidding to extend the one-point gap between themselves and the bottom three.