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.
BBC: Watford's Stephen McGinn set for long lay-off
2.
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.
BBC: Sport - Football - Watford 1-1 West Brom
3.
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.
BBC: Watford boss Malky Mackay hopes to boost squad