Riewoldt went goalless against Carlton in round one but was praised by coach Damien Hardwick for pushing up the ground and playing a selfless team role in "one of his best games for 12 months".
The 2012 Coleman medallist has an outstanding recent record against St Kilda, with bags of eight, seven and six goals in three of his last four games, and the Saints lack a strong match-up for the 195cm goalkicker.
Frawley, who captained St Kilda for a club record 177 games, said James Gwilt, who will miss with a knee injury, would have been the Saints' best match-up for Riewoldt.
St Kilda has regained All Australian defenders Sam Fisher (foot) and Sean Dempster (hamstring) from injury, but Frawley said there was still a clear opportunity for the Tigers to stretch their opponents in the backline.
"I'd be very surprised if Riewoldt doesn't play deeper this week," Frawley told AFL.com.au.
"The situation at St Kilda, while they're all good, competent defenders, there's no real standout who's going to stand up and play in a key post.
"A lot is going to depend on team defence and the supply coming in. That'll be pretty important."
Frawley said Gwilt (188cm) and Jason Blake (189cm) were clearly part of the Saints' best backline, but the club needed to add a power defender who could be pencilled in every week to line up on the opposition's No.1 forward.
Should he remain out of contract, West Coast key defender Mitch Brown shapes as a target at the end of this season after the Saints fell short in their bid to secure him during last year's NAB AFL Exchange Period.
"A good key defender is a luxury like a really good Australian wicketkeeper," Frawley said.
"They're there for 10-12 years and you don't have to worry about them too much, but you sure miss them when they're not there.
"That's been the case with St Kilda since they lost not only Max Hudghton but also Zac Dawson.
"It'll be a cause of frustration for Scotty Watters and it's a tough one where he's going to have to mix and match every week and look at who is suited to who."
Saints forward Rhys Stanley (200cm) has shifted back and is likely to match up on Ty Vickery, who was influential with three goals and nine marks against Carlton in round one.
"Stanley would have him for speed, but Vickery might have a bit more class in the air, so it'll be a good duel," Frawley said.
Tom Simpkin (191cm), who played 19 games last season but was a late withdrawal against Gold Coast in round one, is the Saints' other tall option in defence.
