Brazilian left-back Cassio will miss Friday night's final regular season match for Adelaide United because of a thigh strain.
Reds coach Rini Coolen confirmed the injury but was remaining hopeful that the 31-year old, who has appeared in all but one match this term, would be right for finals.
"He (Cassio) is out for Friday and we'll see how he pulls up in the days after and hope he is ok for the finals, but that is what we have wait and to find out," Coolen said.
"It's disappointing that you lose one of your starting eleven players, now we have to look at solutions on how we cover that."
Adelaide meet the Victory, this Friday, February 11 at Hindmarsh - kick-off is at 7:30pm local time (8:00pm AEDT).