The 24-year-old has undergone an operation on a shoulder injury which definitely rules him out of the European Champions Cup quarter-final between the four-time winners and defending champions Saracens in Dublin on September 19.
Ryan, seen by many as a future Ireland captain, would have faced off against Saracens’ outstanding second row star Maro Itoje.
“Ryan suffered a shoulder injury in training before the recent break and has had a procedure to address the issue,” said a Leinster statement.
“He will be out of action for up to 10-12 weeks.”
Ryan will also be a serious doubt for Ireland’s final two Six Nations matches, postponed due to the coronavirus pandemic, against pointless Italy and fellow title contenders France.
The Irish host Italy at Lansdowne Road on October 24 followed by the tough trip to Paris a week later.
Ireland are in fourth place in the table, four points adrift of England and France but with a game in hand