Roberton, who played 37 games in three seasons with Fremantle, was picked up as a delisted free agent last November and has impressed at his new club this pre-season.
The Saints are hopeful defender Sean Dempster will recover from a hamstring injury in time for the club's season opener against Gold Coast on March 30, but the 2012 All Australian could be sidelined for up to three weeks.
Rhys Stanley could be deployed as a tall defender to shore up an undersized backline, but he is also out for one to three weeks with a knee injury.
Riewoldt said the Saints were lucky to snare Roberton, who wanted to return to Melbourne for family reasons, and he was fitting in well under backline coach Danny Sexton.
"He's got a spot down back that he's made his own in the pre-season and hopefully he can continue that in the early part of the year. I think he is a good chance to play round one," Riewoldt said.
"He's deceptively tall (194cm). I think he could play, probably not on the really big boys, but definitely on a second or third tall."
St Kilda was not tipped by any of the other AFL captains to make the top eight this season in the league's annual poll of club leaders, but Riewoldt was not concerned.
The four-time All Australian said 12 months under Scott Watters should bring improvement as the players adjust to the coach's game plan.
"We saw last year we were able to score a lot more heavily, which is something we addressed," Riewoldt said.
"The flip side was defensively we probably gave up a bit too much.
"Just finding that balance is what we've spoken a lot about over the pre-season and we're getting better at that.
"We've got a really big challenge in front of us but I think we're more than capable."
Riewoldt said the club was excited about its attacking options in 2013, with tall forward Beau Maister also enjoying a strong pre-season.
"The forward line can be a real weapon for us this year," he said.
Stephen Milne (56 goals) and Riewoldt (47) were the Saints' primary goalkickers last season, with Ahmed Saad (28) third.
The club has added medium sized forward Trent Dennis-Lane from the Sydney Swans and WAFL goalkicker Tom Lee from Claremont.
"In previous years myself and Milney have kicked the bulk of the goals, but we've got Saad, (Terry) Milera and Trent Dennis-Lane who are going to bring a lot of excitement and pace to the forward line.
"We've got a bit of versatility up there, which we probably haven't had in the past. We certainly haven't had that depth anyway."
Nathan Schmook is a reporter for AFL Media. Follow him on Twitter: @AFL_Nathan