Michael O'Leary said an initial probe points to foreign object damage during takeoff from Thessaloniki as the likely cause of the July 10 engine failure and window break.
The 18-year-old Boeing 737 had an engine overhauled within the past two years, O'Leary said, adding a draft report is due in under a month.
Engine debris smashed a window about six minutes after departure, forcing the jet to descend from above 15,000 feet to around 6,000 feet and return to Thessaloniki after burning fuel.
A 61-year-old passenger was partly sucked out of the opening before his wife and other passengers pulled him back inside; he was hospitalized with neck, shoulder and friction-burn injuries.
The U.S. NTSB is investigating, and a finding of external-object damage could limit both Boeing's and Ryanair's responsibility for the incident.