Reilly will have surgery which will sideline him between five and six months, Crows coach Brenton Sanderson says.
Reilly was hurt in the opening minute of last Saturday's win against Gold Coast - his comeback match after being dropped and told to lose some weight by coaching staff.
"It has been one of those years for Brent," Sanderson told reporters on Wednesday.
"He has struggled for form a little bit during the season.
"And we got him back, training hard, he dropped a couple of kilos, stripped off a bit of extra muscle mass - it wasn't that he was overweight.
"We feel really disappointed for him."
“It’s about a six-month injury, so like Taylor Walker, we’ll get Brent ready for next season at Adelaide Oval. I’m sure he’s got plenty of good footy left in him.”
The 29-year-old Reilly is expected to have surgery in the next week.
Reilly's absence is compounded by the sudden retirement of a possible experienced replacement, Graham Johncock.
"It means our backline is even more inexperienced going into this week, but those boys have stood up pretty will this year," Sanderson said.
Patrick Dangerfield (ribs) and Matthew Wright (ankle) are tipped to recover from their injuries and play against West Coast on Saturday night in a game between clubs with six wins and seven losses.
"We need to flick the switch pretty quick. If we are going to be playing our best footy, it has got to happen this week," Sanderson said.
"It looks like if we are going to play finals, we're going to do it the hard way and are going to have to scrap and fight and do everything we possibly can to try and finish sixth to eighth."
