The troublesome position continues to give Merrick a headache after youngster Luke Adams struggled on the right side of defence in his Phoenix debut in last weekend's season-opening loss to the Brisbane Roar at the Cake Tin.
Adams has been ditched from this week's matchday squad altogether, and with Louis Fenton still at least a week from returning from his shoulder injury, Merrick has three options at his disposal for Sunday.
"I've got three choices and I haven't made a decision, "Merrick said after training at Newtown Park in Wellington today.
"I'll decide as I usually do at the last minute so it's between Leo [Bertos], Josh Brindell-South and Albert Riera."
Bertos's chances will depend on how he has pulled up from a busy travel schedule with the All Whites during the past week or so, while he has also been bothered by an infected toe lately.
Riera would be an interesting choice given he seems more at home in the midfield, while Brindell-South is largely an unknown quantity.
Merrick won't move Manny Muscat to right back despite the Maltese international's competence in the position because Merrick doesn't want to break up his midfield of Muscat, Vince Lia and Carlos Hernandez.
Riera ran at right back with the first team during training today but that could simply be because the Phoenix's All Whites contingent of Bertos, Andrew Durante, Jeremy Brockie and Glen Moss have flown straight to Sydney from the Caribbean following New Zealand's 0-0 draw with Trinidad and Tobago this week.
It all makes for a disjointed preparation for the Phoenix but it seems likely that most of their internationals will come straight back in to the starting side for Sunday's game.
Skipper Durante will slot in at centre back in place of Michael Boxall, while Jeremy Brockie will be squeezed in somewhere up front, which could mean bad news for Jason Hicks.
Moss is expected to start in goal pushing Jacob Spoonley, who was impressive against the Roar, to the bench.
The Wanderers, who are only in their second season in the league, are one of the competition's best-supported sides and boast 16,000 fans who bought memberships following their inaugural campaign last year where they made the grand final.
A bumper crowd is expected for this weekend's game at Parramatta Stadium, which has a capacity of 21,500.
"It's their first home game, it's a sell-out crowd and as I've said earlier, I would much prefer our boys played against a full crowd like that rather than three people, a dog and a couple of seagulls," Merrick said.
"So I think you've got a lot of energy from the crowd. It's doesn't matter whether they're for you or against you, there's a lot of energy from the crowd and it's great top see the A-League attracting those types of crowds."
Western Sydney began their season with a 1-1 draw against the Central Coast Mariners at Gosford last weekend.
