Merrick stopped short of suggesting Muscat had the chance to make the left back spot his own for the rest of the season.
"I don't think that far ahead. I don't go from week to week either. I tend to look two to three weeks ahead,'' the Scotsman said.
"I just don't want to concede goals through naievity and slight errors, not just from the youngsters. Manny is certainly going to bolster a strong defence already with Glen Moss in goal and two All Whites centre backs and I thought Louie Fenton did a great job for the All Whites when he came on against Mexico. So I'm really impressed with the way we are going at the back there.''
Caira, who is only 20, started the first six games of the A-League season for the Phoenix but Merrick reasoned he didn't want to run his youngsters in to the ground.
"I've always believed that young players need some time in and some time out and after six games it's time to give Reece a bit of a rest.''
Jeremy Brockie will also miss Sunday's game with a minor hamstring tear and is expected to be on the shelf for a week.
Brockie's withdrawal will provide an opportunity for either Paul Ifill or Kenny Cunningham to start.
The Phoenix remain in ninth on the A-League ladder and are still yet to record a win this season, while Western Sydney are rolling in second place.