The 22-year-old defender wasn’t named in Ange Postecoglou’s 30-man provisional squad on Wednesday after ongoing complications with a knee injury.
Sainsbury fractured his patella in February when he landed on a sprinkler on his debut for Dutch club Zwolle.
The injury was initially expected to keep him out for three to four weeks, but the bone hasn’t healed as hoped and pain has persisted, cruelly forcing him out of World Cup calculations.
"It’s disappointing because the team has finally been announced and I was hoping I’d be right in time for it," Sainsbury told FourFourTwo with emotions clearly still raw.
"It’s not like the World Cup comes around each year.
"If I got taken out in a tackle or landed awkwardly I could cop it, but I just landed on a sprinkler which pisses me off."
When asked how he reflected on the circumstances of the injury, Sainsbury added: "Shit happens, that’s my motto since it happened.
"Everything happens for a reason, but usually you only say that when bad stuff happens. I’ve got to try and stay positive and work on other attributes."
Sainsbury returned to Australia a number of weeks ago and continued his rehab in Sydney where further scans revealed the bone was starting to fuse together.
However, there were lingering complications with the ligament shaved from the impact of the kneecap popping out of place.
The former Central Coast man will now concentrate on his rehab in Sydney over the next two months and said surgery would be a last resort.
"I don’t want to have to go through it," Sainsbury said. "I’ll have to rest until it’s right.
"I’ve got a two-year contract as well, so I don’t want to rush back into things and ruin my chances of having a career."
Sainsbury said he had sought out advice from former Mariners team-mates Matt Simon and John Hutchinson who have both dealt with long-term injuries before, along with ex-Central Coast strength and conditioning coach Andrew Clark.
The Perth-born defender has also spoken at length with his former Central Coast mentor, Arnold.
"I give Arnie the utmost respect," Sainsbury said. "He saved my career when I was coming from the AIS to the Mariners.
"The message is to try and keep your head up. It’s not the end of the world.
"I’m still young, there are other opportunities coming up like the Asian Cup and, in four years’ time, get yourself bigger, better and more experienced for that World Cup."