Sheridan has scored four goals in 13 SPL appearances since joining the club on loan from CSKA Sofia.
Sheridan will return to the Bulgarian side when his loan expires in early January.
“Unfortunately Cillian’s expected to be out for three to six weeks,” said St Johnstone boss Steve Lomas.
“But his bad luck is now an opportunity for the likes of Marcus Haber, Carl Finnegan and Sean Higgins.”
