The former Nottingham Forest player suffered a punctured lung and two broken ribs in a collision with goalkeeper Boaz Myhill during the 2-0 defeat at West Brom on Saturday.
The 27-year-old required lengthy treatment before being stretchered off at the Hawthorns, and he will now be sidelined until mid-December at the earliest.
"The challenge itself was pretty innocuous, but it was knee against ribs and when that happens, there was only ever going to be one winner," Palace physio Alex Manos said.
"Adlene is looking at upwards of six weeks out now. It will take four to six weeks for the injury to heal and then another couple to get back into full training."
Eagles caretaker boss Keith Millen had earlier told Advertiser Sport: "The trouble with cracked ribs is that you can't exercise and you can't get your heart and lungs going.
"Some injuries, you can keep your cardiovascular fitness up, but with this injury you can't do anything.
"In four weeks' time, if he can do some exercise and it's not aggravating him, then great, he can."