Rosa was injured laying a tackle during the third term of the Eagles' 28-point loss to the Sydney Swans at Patersons Stadium on Sunday.
He immediately came from the ground and did not return. Scans on Monday revealed a tear in the right tendon and he will require surgery later this week.
It is a bitter blow for Rosa, who was enjoying a rare injury-free season until Sunday. He had just re-signed for two years after qualifying for life membership when he played his 150th match in the round 15 Western Derby.
He joins Beau Waters (shoulder), Scott Selwood (ankle), Mitch Brown (ankle and knee), and Elliot Yeo (broken hand) on the sidelines as the Eagles attempt to salvage something from the remainder of the year.
Youngster Brant Colledge has also been ruled out for up to a month after injuring his ankle playing for East Perth in the WAFL last Saturday.
