Scobbie has suffered ankle ligament damage and will be sidlined for the remainder of the season.
Frazer Wright is another nursing an ankle complaint and could be out of action for up to a month.
With their options severely depleted, the Saints have been left with little choice but to scour the market for new recruits.
Former Motherwell defender Tim Clancy, who was released by Hibernian in January, is hoping to impress during a trial spell, but Wright will explore all avenues in an effort to plug the gaps in his back four.
He said in the Daily Record: "Frazer still has swelling on an ankle. Tam Scobbie needs an operation and will get that next week and he will be out for the rest of the season.
"We can't go with four defenders for the rest of the season. The only other fit centre-back is Ally Gilchrist, who has played only U20 football.
"If need be we will use him but it would be remiss of me if I didn't bring someone in going into such a crucial part of the season.
"We are looking at one or two options."