"That [groin injury] was one of the lowest parts of my life so far,'' said Brindell-South, who played for Moreton Bay United last season in Australia's National Premier League.
"It really got me down thinking it might mean me missing out. I was on the phone to my mum and dad all the time because I felt so bad.
"It has been the longest six weeks of my life and to have this outcome is fantastic.
"Hopefully I can repay Ernie for the faith he has shown in me.''
Merrick is confident that the youngster can do the job he has been recruited for.
"Josh is a fullback who can cover left and right and his resume is quite impressive for a young Queenslander,'' the coach said.
"He won best and fairest awards at his club and being in the NPL means he has been playing at a good standard from a young age.
"What I really like is that he is a very attacking minded fullback and his fitness testing, now that he has finished rehab, is in the superior endurance level.
"The fact that he came in with a slight injury we didn't know about shows he was determined to impress us.
"I believe it was worth waiting five weeks while he rehabilitated because for a young lad he has a very professional outlook and does all the right things.''
Since Merrick took over at the club he has added youngsters Alex Rufer, Reece Caira and Luke Adams, alongside experienced Costa Ricans Carlos Hernandez and Kenny Cunningham. They have also signed back-up goalkeeper Lewis Italiano.
The Phoenix now have the A-League minimum 20 players on their books for the upcoming season although it's understood the club is close to releasing Cameron Lindsay from the final year of his contract. The young Kiwi has been barely sighted in the past two years and doesn't feature in Merrick's plans.