Steven MacLean could miss the next six months of St Johnstone's season after undergoing knee surgery.
The striker, 32, had an operation on Tuesday, but the "clean up" procedure uncovered an additional problem.
"He worked extremely hard to get back from his injury last season and has been magnificent since he returned," said manager Tommy Wright.
Defender Tam Scobbie could also miss three months, with surgery required to repair medial ligament damage.
He will undergo an operation on Monday.
|Tommy Wright on Steven MacLean|
|This time we just have to support him through what will be another tough period but he will get back|
MacLean missed four months of last season through injury, but has scored three times in seven games this term.
"While the area of the knee affected was not the same area as the one which gave him a problem last season, unfortunately the surgeon needed to do another procedure in different area," Wright told the St Johnstone website.
"The consequences of that is an absence of between four and six months.
"This time we just have to support him through what will be another tough period but he will get back."