The 27-year-old punctured a lung and suffered two broken ribs during the defeat at West Brom on Saturday.
The former Nottingham Forest midfielder is expected to be out of action until mid-December at the earliest.
"It doesn't look good for him - he's fractured two ribs and it's damaged his lung as well," Crystal Palace caretaker boss Keith Millen told Advertiser Sport.
"For cracked ribs, you're looking at around at least six weeks out.
"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."