Mellor, 23, would have been part of Kieran Purtill’s Knights camp last autumn, but for scheduled surgery on a shoulder injury that had bugged him for the second half of the season.
He said: “Everything’s sorted with my shoulder. I’ve been training fully fit for about a month and a half now.
“I got the injury half way through last season and I was managing to play on, but I wanted to get it sorted in the off-season.
“I maybe could have played for England Knights but the priority was to get my shoulder sorted.
“If I can have a good season, I can get noticed again and have the chance to play for the Knights.”
Mellor was an integral part of Denis Betts’ side last season, starting in all but one of the club’s 30 games, as the Vikings achieved a respectable 10th placed finish.
He added: “I played week-in, week-out last season and it massively improved me as a player.
“I think the club has strengthened in the right areas this year and the top 8 is definitely achievable.”