Doctors have re-opened the wound and it is being given time to naturally dry out to reduce the chance of infection.
Armitage, 25, suffered the laceration to his left knee against Adelaide last week but he played out the game after having it stitched up.
However, the wound - which was gaping - became red and he was admitted to hospital to stave off infection.
As the Saints beat Essendon on Saturday night at Etihad Stadium, Armitage remained in hospital with his left leg in a brace.
St Kilda head of football Chris Pelchen told AFL.com.au that Armitage was expected to leave hospital in the next 24 to 48 hours and was likely to miss at least two more games.
The Saints play the Brisbane Lions in Wellington on Anzac Day this Friday and then Hawthorn at the MCG in round seven.
They round out their first chunk of the season with clashes against Carlton and Gold Coast before their bye in round 10.
Coach Alan Richardson visited the on-baller on Friday and said it could be some time before he returned to the field.
"He'll be a little while. We're not sure yet; it depends on how his knee pulls up," Richardson said.
"It's not a structural thing. It's just a really bad laceration with a little bit of infection.
"They're on top of it now and he definitely won't play over there [in Wellington next week]."
Armitage shared a photo of himself in his hospital bed on Saturday night as his teammates prepared to take to the field.
Meanwhile, the Saints could have defender Sam Fisher available for selection next week after he got through 80 minutes in the VFL on Saturday afternoon.
However, Richardson said Fisher's lack of a pre-season caused by hamstring and back issues meant he was not a certainty to be called up after just one hit-out.
"I haven't had a look at the game but I've had a little bit of a report," he said.
"'Fish' was good, [Shane] Savage was really good, he responded really well to going out of the team. Maverick Weller, albeit is a rookie, but had another strong performance.
"It's pleasing 'Fish' has got through and is potentially available.
"He hasn't done a lot of work but we'll just wait and see on that one."
Richardson confirmed James Gwilt had raised an initial alarm on the bench when he landed on his hip after being out-bodied mid-air by Leroy Jetta in the third quarter.
After some concern over whether he could finish the game, Richardson said the backman was able to run off the soreness.
