Leinster's Fitzgerald, who was enjoying one of his longest runs in the squad during a career blighted by injury, caught his foot in the ground during training last Friday and suffered medial ligament damage to his left knee as a result.
"He is expected to be out of action for six to eight weeks and at this stage no replacement has been called in," Kearney told a news conference.
"He was very upset. He's had a very good run injury-wise since the end of last season, he had a great World Cup and was flying for Leinster. It's just very unfortunate."
Ireland open the defence of their Six Nations title at home to Wales on Sunday.
(Reporting by Padraic Halpin; Editing by Alison Williams)
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