The Ireland wing requires surgery on a knee injury and could now miss the remainder of the Magners League season.
Fitzgerald ruptured a lateral collateral ligament in his country's 20-20 draw with Australia on Sunday and will undergo surgery today.
The 22-year-old was one of Leinster's star performers as they secured a third-place finish in last year's Magners League and claimed a first European crown.
Fitzgerald was also 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.
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.
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland