The 24-year-old Greece winger appears to have injured his cruciate ligament during the scoreless friendly against Turkey's Sivasspor on Friday.
He is likely to have emergency surgery in the coming days and may not be back before Christmas.
As a result, Panathinaikos’s reported agreements to sign ex-PAOK defensive midfielder Vitolo and Al Saad's right-side winger Owusu-Abeyie couldn’t have come at a better time.
Also on Friday, Panathinaikos confirmed the departure of Djibril Cisse.
The French talisman signed a deal with Italian team Lazio, which should fetch Panathinaikos some 5.8 million euros, according to reports.
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