The Ireland midfielder will be missing for up to three weeks with a hamstring problem.
He's already had groin surgery this season, and tore his hamstring in October.
Koeman says regular injuries are an issue.
Sam Allardyce remains favourite for the vacant Crystal Palace job.
The former Bolton, Blackburn and West Ham boss is being linked to the position following the sacking of Alan Pardew yesterday.
Allardyce was sacked as England boss after just 67 days earlier this year.
