Gardos, who signed from Steaua Bucharest last summer, had hoped to establish himself in the Saints first-team this season following the departure of Toby Alderweireld to Tottenham.
But his setback leaves Koeman seeking at least one new centre-back as he searches for a partner for Jose Fonte at the heart of the defence.
“Very sad news,” said Koeman following Southampton’s 3-0 win at his former club Feyenoord.
“He was going into his second season in Southampton and young, and that type of injury and you lose six or seven months and everybody is disappointed.
“That’s the priority, to bring in one or maximum two centre-backs,” the Dutchman said. “It’s too small with Maya (Yoshida) and Jose (Fonte).
“Ok, we have (Jack) Stephens, we have (Jordan) Turnbull, but they are young and still they have to improve themselves and that’s the point for attention at the moment, the centre-back position.”