A scan later today should determine the full extent of the injury but the initial prognosis is that the 24-year-old is likely to miss up to three months of football. That would sideline him until after January's winter break.
He was substituted after 25 minutes of the 0-0 draw in Paisley following a challenge with St Mirren's Kyle Magennis. Halkett walked off the pitch and was later told by medical staff that he had suffered medial ligament damage.
His form since joining Hearts from Livingston during the summer has been hugely impressive. Indeed, he is the club's top scorer with four goals so far this season and also wore the captain's armband at the weekend with skipper Christophe Berra rested.
News of his absence is therefore a huge blow to the Edinburgh club, who are currently awaiting the return of several first-team players from injury. Those include Steven Naismith, John Souttar, Peter Haring, Jamie Walker, Conor Washington, Ben Garuccio, Craig Wighton and Euan Henderson.