The league gave the Saints permission to take Membrey out of their side on Saturday morning, a few hours before bounce down at the MCG.
He was replaced in the team by Anthony Caminiti.
"Membrey is currently receiving appropriate care and treatment due to a personal health matter," St Kilda said in a statement.
"Tim's health and wellbeing is our primary concern and the club asks that Tim and the Membrey family's privacy is respected at this time."
Membrey had returned for St Kilda's final three home-and-away matches after overcoming a knee injury, kicking three goals in the round-24 loss to Brisbane.
The experienced forward had been poised to line up in attack alongside fellow tall Max King for just the third time this season.
But Membrey's unavailability is a blow to the Saints' chances of winning the knockout final.
The 29-year-old had recently spoken of his excitement about playing in a finals series for just the second time in his decade-long career.
"It's been a bit of a frustrating year personally but we finally got there, the knee's feeling great and the body's feeling good," Membrey told AAP last week.
"I'm three games in and my fitness level and everything is up to scratch, which is good.
"I'm hitting all the numbers I need to hit and I'm as confident as if I'd played the whole year."
A crowd of more than 65,000 is expected for St Kilda's first September appearance in Melbourne for 12 years.
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