Mature-age St Kilda recruit Tom Highmore has shot into round one calculations as the Saints prepare to shuffle their defensive half in the wake of the club's latest injury setback.

Highmore, who turns 23 on Wednesday, stood out playing across half-back for the 'A' side in the Saints' 18-versus-18 match simulation on Thursday.

Coach Brett Ratten suggested the 193cm South Adelaide product could develop into playing a "second or third" tall role in defence after being taken at pick No.45 in the 2020 NAB AFL Draft.

Veteran defender Jake Carlisle played in the opposing 'B' side on Thursday, while ex-Hawk James Frawley watched on recovering from hamstring tightness.

It leaves spots up for grabs alongside Dougal Howard and Callum Wilkie with important backman Ben Paton to miss the entire season and Jarryn Geary at least the opening month of the campaign.

Ratten refused to guarantee Highmore a berth in round one but acknowledged there was an opportunity there to be taken.

"Young Tommy Highmore is nearly a Wilkie clone in the way he goes about it but he's a right-footer and he played exceptionally well mopping up," Ratten told after Thursday's hitout.

"He kicks the ball well, he's brave, he's a good size… so whether he's a second-tall defender or third he's sort of in that category.

"Is he going to play round one? We're not sure. Is he going to play this year? We're not sure.

I had a one-on-one meeting with him the other day and we spoke about that, it's about getting your opportunity and taking it and he's up for the challenge.

- Brett Ratten

 "At 22, he's a long-term view too, for us. Whatever he does this year will be a bonus because he will improve, he will adapt to the AFL (and) the way it's played.

"He's already mentioned, 'Gee it's a bit quicker, if you don't get your positioning right you get caught out'. And I'm like, 'Exactly'."

Highmore was named in the 2019 NEAFL Team of the Year playing for the Canberra Demons before crossing to the SANFL last year.

He will get another chance to stake his claim when the Saints take on North Melbourne in a practice match on Thursday before their AAMI Community Series date with Carlton on March 4.

Ratten forecast the potential of Ben Long playing a deeper role to cover Paton's loss with Dan McKenzie and Jimmy Webster other options in defence.

Swingman Josh Battle impressed as a forward on Thursday, but Nick Coffield appears a lock in defence to start the season.

The Saints are preparing to roll a series of players across half-back this year, with Seb Ross and Jack Sinclair – a player Ratten said was best-on-ground – used there for the 'A' side on Thursday.

It is expected Bradley Hill will rotate through half-back at times along with Hunter Clark and ex-Swan Zak Jones who is touch-and-go for round one after a recent hamstring strain.