Mackay was due to return to training on Friday from the calf injury which ruled him out of Sunday's win at Aberdeen.
Striker Peter MacDonald is also back in training following the knee injury which has sidelined him for more than two months, and full-back Danny Grainger returns from suspension, but Stuart McCaffrey (groin) is still out.
Saints (from): Smith, Duberry, Grainger, MacKay, Anderson, Morris, Craig, Swankie, Davidson, Morais, Millar, Deuchar, Sheridan, Samuel, Rutkiewicz, Main, Irvine, Reynolds, Falkingham.
Source: Team Talk
Source: Team Talk