Michael Smith has suffered a flare up of his back problem and Hearts are unsure when they will be able to welcome him back.
The versatile defender sat out the trip to St Johnstone last Saturday, and boss Robbie Neilson has now revealed that was due to the issue reoccurring.
He will definitely miss St Mirren tomorrow but no timeframe has been placed on his recovery.
Neilson told the Edinburgh Evening News : “Michael is still out. He had a wee issue with his back which we thought was sorted but it flared up again. We are just letting it settle down.
“I couldn’t put a timeframe on it because it’s one of those things that just needs to settle.
"He had an injection and we felt that would settle it but something else flared up off the back of that. It’s just one of those things.”
January signing Nathaniel Atkinson was brought in to initially play understudy to Smith, but has had to feature more often than expected due to the injuries.
The Australian scored at the weekend and Neilson has been pleased with how he's fared, and feels there will be more improvement to come.
“Nat has played a bit more than we expected,” said the Tynecastle boss. “We knew he was coming from the other side of the world, hadn’t played for a bit, was coming into different football, a different environment and different weather.
“To his credit he has done well and I think there is a lot of development to come from him.
"The plan was that Michael would be the first-choice and gradually Nat would come in towards the summer. He’s had to play more than planned.”
