Called up to the international set-up despite just three appearances for Leinster this term, Fitzgerald made appearances off the bench against Scotland, France and Italy but he lasted just 12 minutes in Rome before limping off.
And Leinster revealed in a statement that the 25-year-old is unlikely to feature again this season, confirming Fitzgerald had ruptured the anterior cruciate ligament in his left knee.
"Luke will now require surgery once the swelling settles down and will not return before the season ends," read the statement.
There was however better news on centre Gordon D'Arcy, who is expected back in action soon, after picking up a foot injury on international duty against England.
The statement added: "Gordon D'Arcy continues to make good progress with the foot injury which side-lined him for the last month and the influential centre is expected to return to action either this week or next."
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