Kelly was substituted out of the Giants' 44-point win over Sydney on Friday night and scans have since confirmed a calf strain.
The 30-year-old midfielder was also managed earlier this month with a hip issue.
Tall defender Buckley also has a calf strain out of the Sydney derby and faces multiple weeks on the sidelines.
The pair are among three big names to miss the Giants' huge clash with the Western Bulldogs on Thursday night, with captain Toby Greene suspended.
GWS (13-6) sit sixth, two wins clear of the ninth-placed Dogs (11-8) but with an inferior percentage.
Meanwhile, Collingwood are hopeful midfielder Lachie Sullivan has avoided serious damage with a posterior cruciate ligament injury that could end his season.
Sullivan was subbed out of the Magpies' 36-point win over Richmond on Sunday, while Scott Pendlebury was a late withdrawal because of soreness.
Pendlebury is expected to be fit to face Brisbane in an MCG blockbuster on Saturday night, but Sullivan may not feature again in the Pies' premiership push.
"It's a PCL. Scans will determine how long," Collingwood coach Craig McRae said of Pendlebury's injury.
"We'll just wait and see on that before making too many comments around it.
"But unfortunately for the young lad he had an impact into the ground which caused the PCL (injury)."
Richmond expect star midfielder Tim Taranto will be fit in round 21 after he was a late withdrawal against Collingwood with a calf injury.
St Kilda youngster Lance Collard faces a stint on the sidelines after he was subbed out of the epic six-point comeback win over Melbourne on Sunday with a foot injury and ended the day on crutches.
West Coast young gun Harley Reid will be hoping for a positive outcome from his scans on an injured right ankle, amid fears of a syndesmosis injury.
Reid was helped off the field by two trainers in the dying stages of the Eagles' 49-point loss to Fremantle on Saturday.
"I just chatted to him - he's hurt his ankle," West Coast coach Andrew McQualter said post-match.
"I probably don't have much more information than that.
"You can see Harley put no weight on it as he was coming off, and then he was able to walk off.
"We'll go away, scan it and see."
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