Former Aberdeen captain Scott Severin is set to make his first competitive Hornets start following his free transfer move from the Scottish Premier League side.
Danny Graham, the former Middlesbrough striker, is likely to debut in attack after an impressive pre-season campaign following his summer move from League One outfit Carlisle.
Versatile Slovenia Under-21 international Jure Travner is also likely to feature after joining the Hornets from NK Celje for an undisclosed fee.
Former Watford defender Mackay will take charge of the Hornets for the first time as permanent manager as they start a new era following Brendan Rodgers' departure to Championship rivals Reading over the summer.
Doncaster will include all three of their summer signings in their squad.
Midfielder Quinton Fortune and defenders Byron Webster and Mustapha Dumbuya signed their respective deals at the club this week and are all in contention.
Fortune is hoping to link up with skipper Brian Stock in midfield after the pair impressed in pre-season friendlies and centre-half Webster could make his first competitive start in place of departed player of the year Matt Mills who joined Championship rivals Reading this week.
Rovers will be without Dutch defender Jos van Nieuwstadt, still recovering from a recent hernia operation, while the opening-day clash could come too soon for full-backs James Chambers (ankle) and James O'Connor (stomach).
Sporting Life