Loach missed the 3-0 Carling Cup win at Aldershot in midweek after his surprise call-up to the England squad but will be back between the posts when former boss Aidy Boothroyd returns to Vicarage Road.
Back-up keeper Rene Gilmartin made his debut in midweek but will not be on the bench as he has a knock.
Mackay named a strong side against the Shots and was rewarded as striker Danny Graham scored twice to take his tally to four in two games.
New signing Troy Deeney is likely to be on the bench again as he waits for his full debut following his £500,000 switch from Walsall.
Defender Dale Bennett returned from injury to be on the bench in midweek, meaning Mackay has no injury concerns.