Celtic winger James Forrest faces several weeks out through injury.
The 31-year-old has not featured for the Scottish champions since the 5-0 win over Morton in the Scottish Cup.
He was a noticeable absentee from the bench during the Parkhead side's League Cup final victory over Rangers.
And now Ange Postecoglou has explained the Scotland international's situation.
He said: “James Forrest got a bit of a muscle strain in training so he is going to be out for a little while, which is unfortunate for him.
“He will miss the next period of games probably beyond the international break.”
The Australian also provided an injury update on the rest of the squad.
He revealed that everyone came through the 5-1 win over St Mirren at the weekend okay.
"Everyone else was OK from the weekend ,but I will assess everyone who played on Sunday," he continued.
“It’s three games in six days so we are definitely going to make some changes, whether that’s Wednesday or Saturday, I will see how the squad is.”
Meanwhile, Hearts boss Robbie Neilson has stated he is out to "spoil" Postecoglou's 100th game in charge of Celtic.
The two teams meet at Parkhead during the next round of Scottish Premiership fixtures as the Jambos look for their first win at the venue in over 14 years.
In the two previous meetings this season the Hoops won 2-0 and 4-3 and Neilson has maintained he is keen to avoid a repeat of these results in the upcoming clash.
Speaking ahead of the match, he said: "Ange has done a fantastic job, there's no doubt about that. He's given the team a way of playing and a style of play which is entertaining.
"He has done exceptionally well. But hopefully we can go there and spoil that celebration.
"Defensively you need to be solid. But you also have to have the belief that when you have the ball you can go and hurt them.
"When you watch teams that get some success there then they control periods of the game, they pick the right pass at the right time and the right movement.
"We have to do that. The atmosphere is always electric there and it’s a difficult place to go but it’s one that gives us a chance to challenge ourselves as coaches and also as players.”
