Sainsbury scored the first goal of Sunday's match in Gosford, but there was heartbreak in the dying minutes when he crumpled in a heap as he went to compete for a set piece.
The 33-year-old had twice gone down earlier in the match with calf and knee issues, and he took down a Macarthur player when he collapsed to the turf with the achilles injury.
Macarthur players pleaded for a penalty, but Mariners players soon remonstrated with their opponents for not taking Sainsbury's injury seriously.
A devastated Sainsbury was consoled by support staff as he sat on the turf before being carried off the field.
After the final whistle, there were more angry exchanges, this time between Macarthur players and Mariners staff.
Macarthur veteran Luke Brattan was in the thick of the exchanges, trying to plead his side of the story.
Brattan spoke after the match, sending Sainsbury his best wishes.
"No one wants to see that," Brattan told Paramount.
"I think he said he heard a snap. He's been a fantastic player, and I hope to see him back on the pitch."
Mariners coach Mark Jackson is fearing the worst for Sainsbury.
"Nothing confirmed yet, but it didn't look good. He felt something go in his achilles," Jackson said.
"I thought Trent was excellent today. I thought it was a commanding performance from him.
"To see what happened to him at the end was gut-wrenching."
An unmarked Sainsbury scored the opening goal in the 15th minute with a glancing header.
It marked his first goal for the Mariners since 2013, with stints in the Netherlands, China, Italy, Switzerland, Israel and Belgium sandwiched in between.
Kevin Boli scored the equaliser for Macarthur in the 22nd minute, but Arthur Oliveira De Lima regained the lead for the Mariners six minutes later when his short-range strike made its way through a host of bodies.
Towering Macarthur forward Harry Sawyer scored his fourth goal from his past three games when he nailed a header in the 49th minute, and he had two more headers that went close as the visitors pushed for the much-needed win.
Both sides missed golden chances at the death, but it was Sainsbury's injury and the subsequent fallout that ended up dominating the storyline.
The result leaves Macarthur in eighth spot - four points adrift of sixth-placed Sydney with only two rounds remaining.
Two-time defending champions Central Coast are 11 points adrift of the top six.
