"I just seem to struggle to get our best 11 players on the park consistently,'' Merrick said.
"We just need to strengthen our depth.
"It will be good to sign another player in the January window - someone who can score goals - because I seem to be putting on young players who are not quite ready for it yet.
"Obviously there's a concern there, but I still think we'll make the finals.''
Wellington were disorganised in the opening 30 minutes against Perth, but took a shock 2-1 lead shortly after half-time courtesy of goals to Stein Huysegems and Paul Ifill.
However, their joy was short lived, with goals to Brazilian midfielder Sidnei, defender Michael Thwaite and substitute Adrian Zahra in the final 30 minutes securing all three points for Perth.
Phoenix goalkeeper Glenn Moss was looking for a hole to hide in the 91st minute after fumbling Sidnei's long-range strike and watching it trickle in for a goal.
"I don't need to tell the players which ones played badly. They know how badly they played. They've got to bounce back from that themselves,'' said Merrick, who coached Melbourne Victory to two A-League titles.
Sidnei was the hero for Perth Glory, scoring a brace to lift the club back into the top-six.
Glory coach Alistair Edwards was particularly pleased with Sidnei's first goal - a 20m thunderbolt.
"Before the game he told me: today coach, I will score goal. I promise,'' Edwards said.
"I said `fair enough', and then he scores a cracker like that.''
