The centre-half missed Saturday's 1-0 win over Barnsley with a groin injury suffered while on international duty with the USA.
Rookie defender Dale Bennett, 19, made his first start in DeMerit's place and stands by again while fellow centre-back Adrian Mariappa took the captain's armband.
New recruit Craig Cathcart, who agreed a loan switch from Manchester United on Monday, will not link up with the Hornets until after the trip to Home Park.
Mackay beefed up his midfield against Barnsley by recalling John Eustace and pushing Arsenal loanee Henri Lansbury alongside Danny Graham in attack.
The impressive Graham grabbed his fourth goal of the season to sink the Tykes.