Going into their crunch Saturday night clash against Cronulla at Suncorp Stadium, the Broncos need a victory to snap their dismal seven-game losing streak - the longest losing run by any defending premiers since Wests in 1953.
But while their NRL finals hopes are fast diminishing, they have been boosted by Mariner's return which comes after undergoing surgery for compartment syndrome in his right thigh suffered in round nine.
The condition, which restricts blood flow and can lead to amputation if untreated, raised doubts on whether the 23-year-old would take the field again this season.
But Maguire says Brisbane never counted him out.
"He's flying. It's nice to have Deine back, actually. He's a big part of who we are," the coach said on Friday.
"He just had to work through it, and when you go back to when it actually happened, the doctors worked really hard and fast around that space, and they did a fantastic job."
The Broncos will also have forward Xavier Willison in their squad after successfully seeking an exemption following his concussion against the Sydney Roosters last Friday.
Maguire said he was "really happy" Willison would line up, especially as Brisbane are without stars Payne Haas and Pat Carrigan, who are on State of Origin duty, while second-rower Jack Gosiewski has three fractured ribs.
The trio's loss will leave a massive hole in Brisbane's middle, although it allows VJ Semu to make his first NRL start.
"He (Semu) works really hard, and it's nice to see these players getting an opportunity with what's going on in and around the team," Maguire said.
"He's been training hard and working really diligently around building combinations, so I'm looking forward to seeing VJ out there with Presto (Preston Riki)."
Reece Walsh (Origin) and centre Aublix Tawha (ankle injury) also won't play on Saturday, while Ben Te Kura has been released from the club to pursue his NFL dreams.
Centre Josiah Karapani has been dropped for the Sharks clash following his drink-driving charge, along with other traffic offences from last Sunday morning.
Maguire wouldn't be lured into questions on whether Karapani was on his last chance, but preferred to talk about prop Corey Jensen who won't play for the rest of the year after getting blood thinners to treat blood clots in his lung.
"I really hope he plays again. That's a really unusual one. I don't think I've ever heard of a player having blood clots, but he has jumped on the coaching staff," the coach said.
"He is there with Preston, VJ and Talts (Ben Talty) and lending a hand."
For Cronulla, they are looking to continue their push for a top-four finish, but will be without Addin Fonua-Blake, Blayke Brailey and Briton Nikora due to Origin.
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