Former Motherwell defender Tim Clancy, who was released by Hibernian in January after an injury-hit spell at Easter Road, is among those looking to impress in training.
Both Scobbie and Frazer Wright suffered ankle ligament damage last weekend and the latter is expected to be out for at least four weeks.
All four of Saints' experienced defenders - Gary Miller, Dave Mackay, Steven Anderson and Brian Easton - started in Tuesday's 3-0 win over Motherwell and Saints also have an injury in their under-20 ranks with Gareth Rodger unavailable for four weeks.
Wright told Press Association Sport: "Frazer has still got swelling on his ankle and we haven't got a time frame but I would say it's a minimum of four weeks.
"Tam Scobbie unfortunately needs an operation and will get that next week, and he will be out for the rest of the season. It's bad news but we do know he will be right for next season.
"It's a common enough injury but it was a bit freakish how it happened because nobody was near him. As he was running, the way he put his foot down has caused this injury in his ligaments."
Wright added: "We can't go with four defenders for the rest of the season. The only other fit centre-back is Ally Gilchrist, who has just played under-20 football. If need be we will use him but I think it would be remiss of me if I didn't bring somebody in going into such a crucial part of the season. We are looking at one or two options and will then make a decision."
Saints are looking to avenge a 4-0 defeat for the second time this week. Having turned around a New Year's Day defeat at Fir Park, they now seek to atone for their League Cup semi-final loss to Aberdeen when they face the Dons at Pittodrie in the Scottish Premiership.
"We had already beaten Motherwell before this season but Aberdeen have had the Indian sign over us this season," Wright said. "We haven't scored against them this season. Their two results against Celtic proved what a good side they are.
"But if we show the desire and quality of play that has got us 10 league wins this season, we are capable of beating anybody. Hopefully we can cause them a few more problems but defensively we have to be strong."
- Sports & Recreation
- Frazer Wright
- Dave Mackay
- Steven Anderson