Greenacre was Merrick's second-in-command for two seasons in Wellington, taking over with Des Buckingham eight games into the third when Merrick resigned in December 2016.
Now in charge at the Jets, Merrick has instigated a huge turnaround in guiding last season's wooden-spooners to sit second behind defending champion Sydney FC as the play-offs loom.
In contrast at the Nix, Greenacre has stepped in as interim coach for the third time in five years, this time replacing Darije Kalezic at the helm with six matches of the season remaining.
Wellington has four wins from 22 starts, and has been mired at the bottom of the ladder since mid-November.
One of its wins was against the Jets, a 3-2 upset nearly two months ago, and Greenacre, who still keeps in touch with Merrick, has no doubt the Scotsman will be out for revenge.
"I know a lot about Ernie. He's been a huge influence on my coaching.
"I'm looking forward to the duel. I think the gloves'll be off ... but after that we'll have a beer and chew the fat," Greenacre said.
"Ernie's doing so well going to the Jets - where they are in the league, still playing a good brand of football, and scoring goals.
"It's going to be a real tough test."
Greenacre was looking for the Nix to improve on last week's sub-par effort away to Western Sydney Wanderers, when a 1-1 half-time scoreline ballooned out to a 4-1 loss. Two goals were conceded from set plays, Brendon Hamill punishing Wellington on both occasions.
"Up until then, our game plan had kind of worked. We wanted to stop Western Sydney in their half of the field and at times, the press worked really well," Greenacre said.
"This weekend, we want to be a lot stronger on our defensive set plays. We still had chances despite the heavy score line, so that's positive.
"Hopefully, we can convert one or two of those chances against the Jets."
