"I love playing [at fullback]. I played there in my juniors and a bit during the pre-season, and I guess really I just want to get more time there," he said.

The 22-year-old grabbed his 50th and 51st first-grade tries in the second half, thriving in a new-look spine that featured a returning Adam Reynolds, Cody Walker and at times, both hookers - Damien Cook and Robbie Farah.

"Hopefully that combination keeps growing and we get some more tries out of it," he said.

Johnston has filled in at fullback in the past when Inglis was on Origin duty or injured, but many were sceptical that he was going to be up to the task - but the Rabbitohs junior says he shuts the criticism out, and it showed against Manly.

"I just try and not listen to that at all - I've got to be confident in what I can do and really for me, it was going out there tonight and just trying to prove to myself as much as anyone else that I can play the position.

"He's [Inglis] obviously one of the best in the world, if not the best fullback out there. I tried not to let that get into my mind at all and I just went out there to play the best football I can - Alex Johnston-style footy."

Johnston also praised an "outstanding" Braidon Burns, who filled the now-vacant wing spot, and said it was a shame he couldn't grab a try on debut.

Halfback Reynolds also showered praise on the new fullback, labelling him a "key member of the team".

"The more ball we get him in open space, the better for him - he's got a lot of speed to burn and his choices with the ball were excellent as well".

Johnston's two tries showed his versatility - slicing through the Manly back line on the right-hand side to grab his first, which all but killed off the game, before icing it by showing fantastic speed and agility to dash through and dive on a Farah grubber as it trickled towards the dead-ball line - but he believes his best is yet to come.

"I just try to take confidence that I can grow into the position," he said.