"Leinster Rugby can confirm that Dan Leavy had an initial scan yesterday on a complex knee ligament injury but needs further assessment this week," the club said in a statement.
"He has been ruled out for the remainder of the season and into next season to include the Rugby World Cup."
Leavy, 24, won the last of his 11 caps against Argentina in November after missing last year's Six Nations with a calf injury.
Ireland coach Joe Schmidt was already sweating over the fitness of flanker Josh van der Flier, who is out for the remainder of the season after undergoing groin surgery, but could return for the World Cup.
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(Reporting by Nick Said, editing by Ed Osmond)
