Conor McCarthy will undergo surgery after sustaining a ligament injury in Saturday's match against Livingston.
The Saints defender had to be replaced after 36 minutes in the draw with Livingston after rupturing a ligament just above his ankle.
Jim Goodwin confirmed the 23-year-old will have an operation next week and be out for a number of weeks.
Unfortunately the results of the scan weren't great and we will need to pencil him in for an operation next week," the manager said speaking to the press.
"He has done two or three different things to the ankle. It's not something we think is going to keep him out for three or four months. I would imagine it will be the six-to-eight-week bracket.
"He is a young lad, he has never had any serious injuries. He is physically strong and I would expect him to heal quickly.
"The thought of going into surgery now is not as big a deal as it used to be. We are hoping for as good a turnaround as possible."
Charles Dunne returned to the side in place of McCarthy last weekend and the 28-year-old is now set for his first start since a 0-0 draw with St Johnstone in August when St Mirren face Hearts at Tynecastle tomorrow.
And with Dunne, the manager knows he has a more than able stand-in available.
Charles Dunne came on last week and was excellent. We have a very, very good replacement.
"Physically I think Charles is one of the outstanding centre-backs in the league, in terms of his all-round profile - 6ft 3in, rapid, technically very, very good.
"He started the season very well, picked up a knock and has found himself on the sidelines for a number of weeks because the three lads who have been playing have been very consistent. So Charles has had to be patient.
"He came on Saturday and was excellent. We are very lucky to have him. It's not like I am turning to an 18 or 19-year-old replacement.
"I am putting in a centre-half who has a wealth of experience and has all the ability in the world. Hopefully we can keep the big man fit because he deserves that."
Charles Dunne is likely to come in for the injured Conor McCarthy.
