Brown recently negotiated an early exit from his Erebus contract, which was set to run until the end of next season, to join Triple Eight in 2024.
His defection comes at as time when there is no love lost between the two teams as they battle for both the drivers' and teams' titles.
According to Ryan its the rivalry that has made Brown's decision to leave difficult to take.
“Imagine you’ve got kids, and your kid comes up to you and goes, ‘you know that family you hate up the road? I’m going to go and live with them,” he said.
“That’s how it feels. I can deal with it, but that’s how it feels.
“I’m gutted; he’s like my little brother and it’s like he’s moved down the road.
“But, we’ll get through it, we’ll get on with it professionally, a 100 per cent, and we’ll always be mates.
“It’s just, he’ll be the one I hate next year. Not hate, [but] everybody in pitlane’s my enemy. That’s the only way I can deal with racing and stuff, and making sure that our team is the best team in pitlane.
“So, he’s going to the enemy next year but this year he’s our mate, so we’ll press on like nothing’s happened.
“He’s a fun kid to have around, he’s in the garage today, they’re all having fun, with him, it’s the same team as it was when we left Sydney.
“It’s just, unfortunately, next year he’s not going to be here.”
Brown's move comes after a rapid start to the Gen3 era for Erebus, which has been at least a match for powerhouse Triple Eight this season.
As it stands Brodie Kostecki and Brown are leading the drivers' standings while Erebus leads T8 in the teams' standings.
According to Ryan, Brown's motivation is to be at what is seen as the best "brand" in Supercars, rather than the best team.
“It’s disappointing,” he remarked. “I’m not angry or anything, I’m just disappointed, because we put a lot of effort into Will.
“But like I said to the boys today, once you sleep on it for a bit, you go, he’s gone to a brand.
“We’re the best team in pitlane at the moment, so he hasn’t gone to a better team, he’s gone to a better brand.
“So, we need to do more on our brand, and make sure we get to a point where we are like Triple Eight and we’ve won multiple championships, we’ve got big brands behind us – which we have – and we’ve just got to tick a few more boxes that make the drivers was to stay.
“Will’s made a personal decision and good on him. He could be there for 10 years because that’s the history of Triple Eight. They have drivers for a long time.
“As selfish as you might feel it is on Will’s behalf, [team owner] Betty [Klimenko has] got a beautiful heart and she’s made a decision to let him go, so now we’ll move on.
“He’s still a hundred percent committed to winning races for us. He hates Red Bull, or Triple Eight, as much as anyone right now because he’s with Erebus, so he wants to beat those guys as much as we do.
“At the end of the year, we’ll shake hands and make sure we hate him next year and we keep beating him.”
As revealed by Motorsport.com, Brown sought permission to break contract through Klimenko, rather than Ryan.
Ryan, however, doesn't have any issue with Klimenko 'calling rank', given she owns the majority of the team.
“It wasn’t calling rank. She’s the boss at the end of the day, so she owns Erebus Motorsport. I’m a part owner in a TRC," he said.
“She owns Erebus Motorsport, I run the place, but she can call rank anytime she wants on what she wants to do.
“So, she made the ultimate decision to let him go, [and] I have 100 per cent respect for Betty’s decision.”
