THE MANTRA of 'next man up' was ringing in Harrison Petty's mind.

The young Melbourne defender had been waiting, preparing and ready for his opportunity at senior level. The Demons, keen to reward Petty for his improved form and fitness to start the year, were searching for the chance to give him that moment.

Ultimately, it came about in a manner that no one had hoped for nor expected when Adam Tomlinson's bright start to the year was abruptly ended by an unfortunate ACL injury. But when the opportunity did arise, the 21-year-old was keen to lay claim to a sudden and unforeseen opening in the side.

Petty, who kicked five goals from five games as a lanky teenage forward to finish the 2019 campaign, had missed the entirety of last season after being sent for surgery to fix a debilitating groin injury.

He returned to pre-season this summer and was immediately sent back to his natural position behind the ball, where Melbourne coach Simon Goodwin had been impressed with his improved continuity and his innate ability to read the play.

When an in-form Steven May fractured his eye socket earlier in the year, Goodwin had no hesitation as to who would replace him. The suggestion of shuffling the magnets and shifting Tom McDonald back into a defensive role was immediately shut down. Petty was ready.

The same went for when Tomlinson ruptured his ACL in a victory over North Melbourne a fortnight later. His long-term absence would undoubtedly leave a gaping hole in a hugely important Melbourne backline. But it was a hole that could, and has, been capably filled by Petty.

"You've got to be ready any week," Petty told AFL.com.au.

"An injury can happen whenever, so I always made sure I was catching up with the coaches every week to see if anything had changed in the game plan. I always had my head around that, so when Adam did go down I felt like I was ready to go.

"I knew the game plan clearly and I felt like I could slot in there nicely and keep the momentum going."

Petty returned to a team that was preparing to improve to 8-0 on the season against Sydney the following week. The key position pairing of May and Jake Lever, aided by Tomlinson as the third tall in the group, had provided the springboard for Melbourne's success.

With just 12 senior games to his name up until that point, a significant portion of them spent playing as a forward, you could forgive the young Norwood product – recruited to the club with the 37th pick in the 2017 NAB AFL Draft – for having some nerves.

But he didn't play like it. Petty has since accounted for the likes of Josh Bruce, Eric Hipwood and Brody Mihocek, among others, playing an important lockdown role that enables May and Lever to intercept and attack on the rebound.

In last Thursday night's resounding victory over fellow top-four rivals Port Adelaide, Petty was able to have a similar sort of offensive influence himself. Working off a combination of Todd Marshall and Mitch Georgiades, his eight intercepts were a career-high.

It was a performance that reflected Petty's growing maturity within the side's backline, born from a confidence in his own ability and a summer spent finding valuable rhythm after a season out of the game. It was also one in a series of displays that have meant the Demons haven't lost any fluency, despite Tomlinson's absence.

"I wouldn't say it was daunting," Petty said.

"I know I've done it before, so I knew I just had to play my role for the team and everything would hopefully fall into place from there.

"Before that, I was playing pretty consistent footy in the VFL and I was happy with where my footy was at. I felt like I was ready to come into the AFL team and hopefully perform at the level Adam was playing at and keep the team going."

Tomlinson's role in Melbourne's backline through the season's first two months had been vital and, even after his injury, the former Greater Western Sydney utility has been playing a pivotal part in the club's surge to the top of the ladder.

Along with May and Lever, the experienced Tomlinson has taken Petty under his wing and has become one of his most valued mentors. They watch vision together, talk about upcoming match-ups and look for ways to improve both from an individual perspective and the team's defensive group collectively.

"He's been unbelievable for me," Petty said.

"As a backline, we're a really tight-knit group. We're a team within a team. We'll get together after games, watch my vision … that's me and Adam, but even Steven and Jake join in to help me out too.

"We'll watch all of our vision, see what we can do better, see what we did well and just try to implement better things going forward."

The role both May and Lever have played in Petty's development can't be understated, either. With both in Therabody AFL All-Australian form, the duo have given their young teammate the confidence to turn defence into attack and deservedly hold down an important place in the back six.

"Playing with them makes you feel really good," Petty said.

"They're two very smart footballers. They beat their man and then if their man runs up into congestion, they're smart enough to drop back into the hole and I'll be able to seal my man out and hope they take a mark.

"They give me the confidence to go for the ball. If you think you can mark it, then mark it. If not, spoil it. Some of their contest work … when Steven beats Charlie Dixon in a one-on-one, it lifts the whole team and makes you feel like you're on top of the world."