Mackay is already without defender LloydDoyley, while Andreas Weimann and Danny Drinkwater are both looking to shrug off knocks ahead of Saturday's visit of Millwall.
Watford had already seen LloydDoyley 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 LloydDoyley on the injury list, while captain John Eustace will serve a one-match ban after receiving two yellow cards at Doncaster.