The 32-year-old was sidelined for four months of last season with an injury in the same knee.
The Saints were expecting MacLean to be absent for around a month with his latest problem, but it was announced on Wednesday that the forward will be missing for longer.
"I'm extremely disappointed for the player because he worked extremely hard to get back from his injury last season and has been magnificent since he returned," manager Tommy Wright is quoted as saying by Sky Sports News.
"This time we just have to support him through what will be another tough period but he will get back."
MacLean joined St Johnstone in 2012 following a brief spell with Yeovil Town. body check tags ::