GWS have confirmed that Kelly sustained a hamstring strain in the second term of the Round 10 clash but played out the game, now expected to miss around four weeks of footy as he recovers.
The vice-captain has played nine games for the Giants this season, averaging 28.7 disposals, 4.8 clearances and 5.4 tackles per game.
GWS' Head of Medical James Rahme confirmed the extent of Kelly's injury on Tuesday.
“It's disappointing for Josh, who has had a great season so far,” Rahme said.
“He did remarkably well to play through considering, with scans revealing a strain on Monday.”
Phil Davis has also suffered another setback, suffering a calf injury in the VFL over the weekend.
Davis is yet to play a senior game for the Giants this season after contending with injury, with this latest setback to keep him sidelined for over a month.
“It's a frustrating setback for Phil, who has done everything right to get back on the field in recent weeks,” Rahme said.
“As usual, his attitude has been exemplary and he was a great asset to help the younger players during a tight game even after his injury.
“He's already straight back into his rehab and we're looking forward to his return.”