Fitzgerald ruptured a lateral collateral ligament in his country's 20-20 draw with Australia on Sunday and will undergo surgery today.
Fitzgerald was a central figure in Ireland's historic Grand Slam triumph and then went on to win a Lions Test cap in the series defeat to South Africa this summer.
He started the second Test against the Springboks in Pretoria and played a total of five games on tour.
Fitzgerald featured against the Cheetahs, Southern Kings and Emerging Springboks and was a try scorer in the comprehensive win over the Sharks.
The 22-year-old was also one of Leinster's star performers as they secured a third-place finish in last year's Magners League and claimed a first Heineken Cup crown.
With a recovery and rehabilitation period of between four to six months, Fitzgerald could feature at some point before the end of the 2009/10 season but he will definitely miss Ireland's autumn internationals and the 2010 Six Nations.