Morgan Schneiderlin has been ruled out for the rest of Southampton’s season due to a knee injury sustained last weekend.
The France midfielder was forced off in the 90th minute as Southampton drew 2-2 with Tottenham on Saturday to remain in firm contention for a UEFA Europa League spot.
“It’s the ligaments of his left knee and that’s a shame because he’s out for the last part of the season,” Koeman said.
“He’s a key player so it’s disappointing. It’s a knee injury. It’s not five months but at least four or five weeks.”
Groin issues have limited Schneiderlin to 26 appearances so far this season – a figure he will end the season on due to his latest injury.
Despite being heavily linked to north London rivals Tottenham and Arsenal as a host of key players departed St Mary’s Stadium prior to this season, Schneiderlin has remained and blossomed under Koeman.
His impressive form in a campaign that has seen the south-coast club surpass expectations will do nothing to deter any potential suitors.
Schneiderlin’s own claims in January that he will evaluate his standing at the end of the campaign could lead to another close-season of speculation.
Koeman will also have to make do without winger Dusan Tadic for Saturday’s trip to Sunderland due to a groin injury.