Mooy departed the Premier League side and signed for Chinese club Shanghai SIPG after they activated his £4 million (A$7 million) release clause.
The midfielder and Ryan grew up together in Sydney’s west, both attending Westfield Sports High, and have been teammates in the national team for the past eight years.
Last year, Mooy joined the Seagulls from Huddersfield Town.
“Having that national connection and him being somebody I grew up playing with and against each other, there were quite a few moments and memories last season we created (by) beating certain teams,” Ryan told The World Game.
“It’s quite a cool story and something we’re both going to cherish for a long time. But I guess that’s the industry that we live in.
“Aaron’s thinking of what's best for him and his family and all that. I totally respect his decisions and what he does, I’m always here to support him and fingers crossed we have plenty more memories to make on the international scene as well.
“But I’m sad to see him leave. I know he’s a quality player and he will be missed, but that’s the industry we live in.”
Last month fellow Australian Tom Rogic was linked with a potential move to Brighton, with Celtic after defender Shane Duffy.
The Socceroo and Ryan both started their professional football careers with the Central Coast Mariners in the A-League.
The goalkeeper says he was aware of the media speculation but hasn’t been in touch with Rogic.
“I haven’t spoken to him, I’m not really sure what he’s been up to,” the 28-year-old said.
“I’ve seen he hasn’t really been playing a lot at Celtic, but I wasn’t sure. I saw some report linking him to Brighton, but I didn’t know much about it.
“I don’t pay a lot of attention to those kind of reports, but I know he’s another great player as well.
"Some clubs I’ve played for in the past, I’ve mentioned his name a few times, they saw him as an interesting player but it hasn’t eventuated yet.
“I’ve not had chats with the guys here about him, so I don’t know what the situation is.”
Ryan’s Premier League season kicks off on Tuesday (AEST) with his team hosting Chelsea.
With Mooy exiting to China, the shot-stopper will be the only Australian plying his trade in the English top flight in 2020-21.
It’s a far cry from 15 to 20 years ago when the Socceroos had nearly a full XI in the Premier League. Ryan hopes he can inspire more clubs to take a punt on Aussie talent.
“I hadn’t even thought about it [being the only Aussie]. Those are things that are just external, I just concentrate on what I can control myself and the best I can do,” he said.
“If I can help represent Australia in a good way and give the respect that the nation deserves, maybe get some other clubs to take notice of Australians and provide some opportunities for others to play at the highest level as well, then that would be an added bonus.
“Fingers crossed we can get more and more Australians playing at the highest level to give ourselves the best chance for success on the international stage.”
The Seagulls finished 15th last season, avoiding relegation again, and now the Sussex club want to build on that campaign.
“There’s nothing better. When you go out and achieve your collective goals as a team, obviously in this environment, there’s no better feeling,” Ryan said.
“The club that we are, the challenges that we face to maintain our Premier League status, it’s always nice when you can go and achieve that come the end of the season.
“There’s a few cherries on top from last season with it being the best points tally we’ve had so far.
“Given all the changes at the beginning of the year – the big one with the manager change – and all the other little changes that brings with the way we want to play and his philosophy, and the adaptation we all have to make, it can be a bridge too far to climb sometimes for other teams.
“So I think we can be pretty proud as a club to have been able to achieve what we did last season and (we're) now looking to build on it.”