The premiership player and two-time All-Australian hurt her left knee in Saturday's second term at Casey Fields, souring the Demons' 13.11 (89) to 2.3 (15).
Demons forward Tayla Harris kicked a career-best four goals in the rout and Tyla Hanks had a game-high 28 possessions.
The game was already effectively over, with Melbourne more than five goals up, when Purcell tackled Georgia Patrikios.
As Purcell tried to bring her opponent to the ground, her left knee gave way in the twisting motion.
She immediately left the ground and will undergo scans. Purcell suffered a right ACL tear in early 2021 while playing for Geelong.
"We will just hope for the best, that it's nothing that will keep her out for too long," Melbourne coach Mick Stinear said.
Purcell suffered a serious facial injury last year that needed surgery, but returned to play out the season wearing a face mask.
As the Demons flexed their considerable muscle to start the season with a second-straight win, it was particularly disappointing for the Saints given they had opened the season by upsetting Adelaide.
All Harris' goals came in the first half and she could have had an even more prolific game, finishing with 4.4.
Melbourne had seven scoring shots to one in the first term, but a wasteful 2.5 meant the game was still close on the scoreboard.
Purcell was prominent in the opening quarter, with 14 possessions.
The Demons piled on 5.3 to one behind in the second term to remove any doubt.
Captain Kate Hore had 14 score involvements among her 26 possessions, also racking up five clearances and kicking a goal.
Saints midfielder Tyanna Smith laid 10 tackles and had six clearances, also amassing a team-best 20 disposals.
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