However, Murray Davidson should be fit for the visit of Kilmarnock this weekend after recovering from a calf injury and on-loan Brian Graham returns having sat out last Saturday’s win over parent club Dundee United.
Manager Tommy Wright rued the loss of Lappin, telling the Courier: “There’s some damage to the ligaments and there is bone bruising from the contact made in the challenge. It’s a pity because once he got up to pace Simon has been a regular and done well for us.
“Fortunately we have Murray Davidson coming back from his calf strain and David Wotherspoon wants to play in there as well. We still have a great opportunity to make the top six again. We have Dundee and Hamilton games coming up. We feel we have winnable matches before the split.”
Kilmarnock will be without Mark Connolly and Jamie Hamill for the trip to Perth with both set to miss the remainder of the season.
Defender Connolly has a broken collarbone, while midfielder Hamill has undergone ankle surgery and joins Michael Ngoo (ankle) on the sidelines.
Playmaker Alexei Eremenko returns from suspension while striker Lee Miller (knee) could also make the squad after six weeks out.