MELBOURNE recruit Sam Frost is possibly the best athlete his teammate Tom McDonald has ever seen in the AFL system.

Frost was traded from GWS to Melbourne in exchange for pick No.23 (the Giants sent selections 40 and 53 back to the Demons) in last year's NAB AFL Draft, after Melbourne identified the need to bolster its key position stocks

Frost, the younger brother of Collingwood defender Jack, played 21 games in three seasons on the Giants' list and showed plenty of ability despite limited opportunities.

McDonald said he couldn’t be more impressed with his 194cm, 93kg teammate’s physical attributes.

"Sam's probably the best athlete that I've seen in the AFL, bar maybe (North Melbourne's) Majak Daw," McDonald told AFL.com.au.

"He's got incredible speed and strength and he's very strong for someone who's only 21.

"He's definitely got a lot of talent and I think we'll work really well together."

McDonald believes Frost could be an ideal replacement for former Demon James Frawley, who crossed to Hawthorn as a free agent at the end of last season.

The duo has spent plenty of time together this pre-season, working on their defensive techniques and ensuring they can impact the ball in marking contests.

"We're working together and talking to each other every session. He picks up things that I can work on and I do the same for him," McDonald said.

"He's a good learner and he wants to get better. That's the most important thing."

McDonald himself is still only 22, yet he is now Melbourne's undisputed key backman.

The emerging defender coped well with the added responsibility in 2014 and is working hard on improving again this season.

He doesn't have the added distraction of rehabbing his shoulder – which he did last pre-season after having it reconstructed at the end of 2013 – and is in far better shape because of it.

"It shocked me how much I missed out on, even though I was able to come back and play by round one," McDonald said.

"I really was starting behind the eight ball."

Exciting defender Sam Frost works hard at Melbourne training. Picture: Getty Images

Regardless, McDonald plans to stamp himself as more of an offensive threat this season. It's been a demand placed on him from the Demons' coaching staff and driven home by backline coach Jade Rawlings.

"For me, I'm going to have to find a way to play on the best forwards and shut them down and then still have an impact in winning the ball back for us," McDonald said.

"I'm really working on my decision-making and taking that first easy option so I can help the team out that way."