While McStay has avoided a second reconstruction, it's still a significant blow to the Magpies.
He was subbed off after suffering a heavy knock in a marking contest during the third quarter of Friday's Anzac Day win over Essendon.
The injury was to the same knee he had reconstructed at the end of 2023.
Collingwood confirmed on Saturday that McStay had suffered a medal collateral ligament (MCL) strain that will not need surgery.
"Dan's got a grade-three medial strain," coach Craig McRae told the ABC.
"It's about a six to eight-week injury - no operation required, just some rehab and in a brace for a period of time."
There were fears that McStay had re-injured the anterior crucial ligament, or ACL, and that could well have meant a season-ending reconstruction.
McStay missed the Magpies' 2023 premiership win because of a right MCL injury in the preliminary final the week before.
He then ruptured his left ACL that December in pre-season training. He returned from his reconstruction in round 20 last year and played the last five games.
McStay had played in Collingwood's seven games so far this season, kicking eight goals.
Collingwood captain Darcy Moore remains in doubt for the Magpies' clash with Geelong next Saturday as he battles vertigo symptoms related to an inner ear issue.
Superstar Nick Daicos, aged 22, made his Collingwood captaincy debut on Anzac Day as a replacement for Moore.
"My last report was he's improving," McRae said of Moore post-match.
"Still getting some vertigo signs.
"He's on antibiotics, and it's day to day at the moment.
"I think he's improving, still not enough to train, so hopefully that improves over the next couple of days."
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