Roberts, 21, suffered the right knee injury when she attempted to change direction and planted her right leg in the first quarter of Saturday's loss to Hawthorn, the Eagles' final game of the campaign after failing to make finals.
The midfielder immediately went down in pain grabbing her right leg and had to be assisted from the field.
The Eagles confirmed on Monday that Roberts had torn her ACL and will require a knee reconstruction.
Given the typical 12-month recovery from that surgery, Roberts will be racing the clock to feature at all next year.
"We are all saddened by the injury sustained by Ella," West Coast AFLW boss Michelle Cowan said.
"Our priority is to arrange surgery as soon as possible, and ensure she receives the very best care and we will wrap our arms around her throughout this journey.
"She is a true professional, and we have every confidence she will approach her recovery with the same diligence and determination, fully supported by our medical team."
Homegrown superstar Roberts, who can play both midfield and forward, is one of the most talented players in the game and has kicked 20 goals in 55 games since her 2022 debut.
The 21-year-old is a two-time All-Australian and won the Eagles' best and fairest award in 2024 and 2025.
Roberts averaged 21.7 disposals and five clearances this year as West Coast finished 13th.
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