Saints will be without Murray Davidson, who has an ankle problem, as they look to cut the four point gap their opponents hold in the race for a Europa League qualifying spot.
Fellow midfielders David Robertson and Jamie Adams remain out of action with long-term injuries as Saints look to get back to winning ways after the 2-2 draw with Ross County last week.
Caledonian Thistle will definitely be without defender Gary Warren when they make the trip to Perth.
The centre-back returned from a knee injury ahead of schedule against Celtic last Sunday, but was forced off early in the second half of the 4-1 defeat.
Goalkeeper Ryan Esson and defender Simon King are also missing, but centre-back Chris Hogg is pushing for a start after a serious knee injury.