The versatile back, who turned out as both a centre and a winger during the World Cup, was set to play for his country during the upcoming Six Nations but has now been ruled out for the tournament.
The medial ligament damage is the latest in a string of injuries suffered by the 28-year-old and Ireland team manager Mick Kearney admits it was an incredibly unfortunate situation for Fitzgerald.
"He was doing a tackling drill at the Aviva (Stadium) on Friday in the open session and his foot just got caught in the grass and twisted his knee so fairly innocuous," said Kearney.
"He's obviously very unfortunate to be out of the Six Nations. I suppose it could have been worse as well.
"He was very upset, to be honest with you and probably feared the worst. He has had a very good run injury-wise since the end of last season.
"He had a great World Cup and was flying for Leinster right through to the opening session on Friday so it's just one of those things."
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