The injury to Haas is a mega blow for the men from south of the Tweed as they head to Suncorp Stadium - Haas' club home ground - to try and retain the shield with the toughest task in Origin - winning a decider in enemy territory.
But it's not a challenge Fittler believes is past McLean, who will become one of Origin's oldest debutants when he runs onto the hallowed turf in Brisbane aged 30.
While other options like Reagan Campbell-Gillard - who was dropped after Game 1 - or Newcastle Knights' in-form enforcer David Klemmer - who hasn't played Origin since 2019 - were considered strong chances, it was McLean who ultimately won the race to be named.
It comes after he was included in the 22-man squad for Game 2, which was played in Perth and won by the Blues in a big way as they rebounded from a Game 1 horror show at home in Sydney.
Fittler told reporters that his form, as well as that of his team at the Cowboys, is a big part of the reason he has been picked.
"A few years ago I remember talking to a few players in a room and he was one of those young players that was about to make his debut and he never quite got there," Fittler said.
"To watch what he's done this year, he's been outstanding. What he's been doing for the Cowboys - he's an unsung hero.
"(It shows) if you get to 30 - it's never too late. His form, along with the North Queensland team hasn't been great over the last couple of years. (But) the whole club has been unbelievable (in 2022) and he's a big part of that."
Fittler also mentioned that it was the stats jumping off the page for McLean which were a convincing factor in his ultimate selection for the side, and it's difficult to argue, with McLean tackling at 97 per cent efficiency and making 116 metres per game.
McLean will join Jake Trbojevic - who was superb after being recalled for Game 2 - in the front row, with Junior Paulo named off the bench in the decider, to be played on Wednesday, July 13 in Brisbane.