The midfielder suffered two broken ribs and a punctured lung during Saturday's 2-0 defeat to West Bromwich Albion last weekend.
"It doesn't look good for him. He's fractured two ribs and it's damaged his lung as well," interim manager Keith Millen told the Croydon Advertiser. "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."
The 27-year-old joined Palace from Nottingham Forest in August.