Mackay is already without defender Lloyd Doyley, while Andreas Weimann and Danny Drinkwater are both looking to shrug off knocks ahead of Saturday's visit of Millwall.
Watford had already seen Lloyd Doyley and Will Buckley limp off, and they suffered a further setback when proceedings were halted while Cleverley received lengthy treatment.
But midfielder McGinn is likely to join defender Lloyd Doyley on the injury list, while captain John Eustace will serve a one-match ban after receiving two yellow cards at Doncaster.