"I can't split them," said Carle, who was playing for Newcastle Jets when he won the award in season 2006-2007.
"It's a tough one. They're different kinds of players - Mooy controls the game so well and Fornaroli is the type of striker everyone wants.
"Fornaroli is deadly in front of goal, but not just that, his work-rate is immense and he's a fighter, he's got a big heart. Mooy makes things happen for other players and scores goals himself. He runs the game for City.
"It's a very tricky one to judge, because they're both very talented in their own way and each is a pleasure to watch. They've both been brilliant this season."
Mooy and Fornaroli each have sensational statistics this season.
Fornaroli has played 24 games and is the competition's leading goal-scorer, with 20, which is an A-League record. He also has four assists and 78.9 per cent passing accuracy.
Mooy has played 22 games and scored 11 goals from midfield, but his most stunning feat is his league-leading 18 assists. Despite being under constant pressure from opponents who are trying to shut him down, Mooy has 81.4 per cent passing accuracy.
There are several other players who have had tremendous seasons, most notably Mark Bridge, from Western Sydney Wanderers, Brisbane Roar pair Corona and Jamie Maclaren and Perth Glory's Diego Castro.
But as well as they have performed, it is hard to see any of them knocking both of the City players out.
"Bridge has been outstanding for Wanderers," Carle said. "Mitch Nichols started the season on fire for them and has continued to play well, but I think 'Bridgey' has overtaken him as their best-performed player.
"Corona and Maclaren are each right up there as well and Castro, for me, is the one who has made the big late run. Since Perth turned the corner with their form he's been a massive part of their success.
"I know Castro had a lot of critics early in the season, but there was always something about him and he has been dominant recently. But will it be enough? Did he collect enough points at the start of the year, when they weren't doing so well?
"We'll have to wait and see if he has been able to catch up, but any of the players mentioned here would be worthy winners. If you're asking me to narrow it down, though, I reckon the winner is going to be one of the two City boys. I just can't narrow it right down to one."
Carle, who was a highly-skilled playmaker himself, is one of the judges in the revamped process. The medal was once voted on by the current players, but now the voters include a selection of former players, match officials, FFA technical staff and media.
"It's great to be a small part of it," Carle said. "You get nominated a game to vote on every few weeks and you give them your 3-2-1s.
"It's usually your more attack-minded players who win the award, which a bit unfair on the rest of the team because without 11 players out there you're not going to get anywhere. It really is a team effort.
"A number 10 or a nine can have a big impact, but there are other players who can turn a game as well. But it's still hard to go past Fornaroli or Mooy this season and it's easy to see why City are pushing for the title when they have both those players in their side.
"Fornaroli is brilliant for the league. For mine, he's one of the best foreigners to have come here and I know we've had a lot of good ones. Thomas Broich is one who has become a mainstay in the A-League and hopefully we can hang on to Fornaroli for a few years as well.
"I'd love to see the vote count at the end, because I think it would be very interesting, whichever way it goes. I'm thinking it has got to be very close, in which case the last three rounds could well make the difference."
