Graham - who scored four goals in the first two games of the season - was left on the bench for the midweek Carling Cup defeat to Notts County but came on for the final 15 minutes as the Hornets looked to find an equaliser.
But Graham pulled his groin in having a late shot on goal and is a serious doubt to face Leeds.
Mackay was also without captain John Eustace, Lee Hodson and Martin Taylor in midweek as they recovered from niggles and he is hoping they can all return against Simon Grayson's side.
England Under-21 goalkeeper Scott Loach will also return to the starting line-up after he was rested against County.
Former Middlesbrough midfielder Josh Walker returns after being cup-tied on Tuesday night, while on-loan Birmingham man Jordon Mutch is also available again.
