Essendon co-captain Bonnie Toogood will miss the next six to eight weeks after scans confirmed an MCL injury for the star forward.
Toogood strained her MCL in an incident with young gun Amber Clarke in Saturday's loss to Fremantle, where the pair collided in a nasty clash inside forward 50.
Clarke has entered concussion protocols but has been cleared of any neck or facial injuries.
"It's extremely unfortunate for Bonnie and Amber to have to spend time on the sidelines. The incident was a heavy clash between the two players, with both showing extreme commitment to the ball," Essendon head of medical services Megan Smith said.
"While we are disappointed that Bonnie will be out for a significant part of the season, we are relieved that there is no damage to her ACL. Bonnie will commence rehab with the aim of returning later in the year.
"Amber has entered concussion protocols and is recovering well. She was up and talking after being stretchered off and was further assessed in hospital Saturday night to clear any concerns of further damage. Amber's symptoms will be monitored closely, which will determine her return to play."
The Bombers head west to take on West Coast on Saturday.