The highly-rated youngster will be unavailable for several months after picking up a serious knee problem in Friday's Euro 2012 qualification victory over Israel
The 21-year-old picked up the knee problem in Greece's 1-0 win over Israel in Friday's Euro 2012 qualifier.
Ninis scored the crucial goal that helped Greece to the full three points, but was forced to leave the pitch after 77 minutes of play due to the injury.
The Panathinaikos star underwent further examination at the weekend to clarify the extent of the knock and a scan has revealed that Ninis has torn his cruciate ligament.
Ninis will consequently miss Greece's three remaining Euro 2012 qualifiers due to the injury lay-off, while he will also be forced to watch the biggest part of the 2011-12 Greek league campaign from the sidelines.