Alex Johnson's latest setback shocked the footy world and those closest to the Swans defender have rallied around him.

Johnson is set for a sixth knee reconstruction – the first on his 'good' right knee - after rupturing his ACL in the opening term of Sunday's win over Melbourne at the MCG.

It was just his second senior game since the 2012 Grand Final, after he made an emotional return against Collingwood in round 21.

One of Johnson's closest mates, GIANT Adam Tomlinson, was watching the game from his couch in Sydney after arriving back from Canberra after his side's win over Adelaide on Saturday night.

Their friendship stems from junior footy days when they played together for the Oakleigh Chargers and Vic Metro.

Tomlinson took particular interest in the Demon-Swans match because another of his close friends, former Giant Dom Tyson, was lining up for the Dees.

He told AFL.com.au that he'd sent Johnson a good luck message pre-game, and was looking forward to watching the contest before his great mate went down.

"It was devastating and heartbreaking to see it unfold," Tomlinson said.

"I just couldn't believe it and I knew it was his opposite knee.

"I saw him sitting on the bench with the look of disbelief on his face.

"All I wanted to do was pick up the phone and speak to him.

"I sent him a message pretty much as soon as it happened and weirdly he responded pretty quickly, so I spoke to him via text for the next 10 minutes, but I had no idea what to say. 

"I've spoken to him twice today (Monday) but you can't ask how he's going, you just want him to know that you're thinking of him.

"Last week was a bit of closure on the tough last six years he's had, and once it was done he was looking forward to getting back into the groove of things.

"It was his opportunity to get back to life as a normal AFL footballer.

"It's been pretty heavy and emotional since it happened."

Before Sunday's horrible setback, the mates were excited at the prospect of facing each other on the AFL stage in this week's Sydney derby at Spotless Stadium on Saturday.

They hadn't done so since Tomlinson's debut – the GIANTS' first game in the competition – at ANZ Stadium in round one 2012.

"Even before he was back in the senior side I was telling him he was a genuine chance to play against us at Spotless, and it was all shaping up that way," Tomlinson said.

"To be honest every time we've played on each other in the AFL he's wiped the floor with me, so I owe him a few.

"I remember back in 2015 I was playing in the NEAFL and it was his first game back from one of his surgeries, in the first five minutes I didn't get a touch, but he had five or six and kicked a goal on me.

"Five minutes later he did his knee again, but even that was a glimpse that he hadn't lost his ability.

"We had spoken about playing each other this week but unfortunately it's not going to happen."

Tomlinson has often spoken about the homesickness that gripped him when he first arrived at the GIANTS, and at times threatened to see him request a trade back to Victoria.

He said Johnson's presence, even if it was at the GIANTS' bitter crosstown rivals and battling his knee issues, got him through plenty of tough times.

"Since I've been in Sydney Johnno has been a massive support for me, and I've been a lot more comfortable here than I would have been without him," he said.

"Over the last six years every time I've run out onto the field I felt as though I was playing for my family and my club, but at the same time, for Johnno, because he can't be out there.

"I know it's not much to the people on the sidelines, but you always remind yourself how lucky you are."