The Frenchman suffered the problem during the 2-2 draw against Tottenham last weekend, with scans revealing the problem was serious enough to sideline him for up to a month.
Koeman said at a press conference: "Morgan will be out until the end of the season, for four or five weeks."
Southampton head to relegation-battlers Sunderland on Saturday looking to maintain hope of European qualification, currently in seventh place but only a point behind both Spurs and Liverpool.
Source : PA
Source: PA