There were initial fears McStay would be out of action for a longer period when he hurt the ankle during Saturday night's win over West Coast.
But the 26-year-old goal-kicker was cleared of a break and follow-up scans have revealed a sprain.
"He's got a bit of ankle damage there which is an ankle sprain, so we'll give him about three weeks," Lions high performance manager Damian Austin said.
"We'll hopefully get him up and moving by the end of the week."
Eric Hipwood is a chance to return from a long-term knee injury for Brisbane this Saturday night against Adelaide, while fellow tall target Joe Daniher (shoulder) is sidelined for at least another month.
Flag rivals Geelong have lost ruck-forward Esava Ratugolea for several weeks after he underwent ankle surgery on Monday.
Cats trio Rhys Stanley (ankle), Joel Selwood (corked leg) and Shaun Higgins (general soreness) are all hopeful of proving their fitness for Saturday's clash with St Kilda.
But Jed Bews is unavailable after suffering concussion in a collision with GWS forward Jesse Hogan.
Melbourne ruckman Max Gawn is in doubt for the match against West Coast after suffering a knee injury.
The in-form Demons captain had scans on Monday after damaging his knee during the round eight win over St Kilda.
"I didn't do much in the second half," Gawn told reporters on Monday.
"I felt a little bit sore in the second quarter but I managed to play out the half and I've woken up pretty good this morning."
Gawn has battled knee issues throughout his career and does not believe the latest issue is a dreaded PCL injury.
"I've had a pretty good start to the year ... and yesterday just got me a little bit in the second quarter," Gawn said.
"But I played out the half, so I'm pretty confident.
"I'll be pushing (to play this week).
"I've woken up really good this morning and I didn't want to do this (MRI scan) today but they said to go in and make sure."
Young star Luke Jackson did most of the ruck work in the second half against St Kilda is capable of playing the lead role for Melbourne if Gawn is ruled out.
The Demons meet bottom two sides West Coast and North Melbourne over the next fortnight before a clash with Fremantle, who currently sit second.
Dockers forward Sam Switkowski has recovered well from a head knock but will miss Sunday's clash with Gold Coast, while defenders Alex Pearce and Luke Ryan have been cleared of minor injuries.
The Western Bulldogs expect to regain captain Marcus Bontempelli for Friday night's clash with Collingwood but have lost Cody Weightman for three weeks after he underwent surgery on his broken collarbone.
Laitham Vandermeer will miss about eight weeks with a hamstring injury and Tim O'Brien (calf) is also set for a stint on the sidelines.
Carlton have lost Jack Martin for at least a week because of a calf strain.
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