Mourinho's personal bodyguard was wounded as the madman lashed out with the sharp object.
Jose Mourinho, was rushed away by his security team at the A Coruna airport, in north west Spain, last Friday.
Spanish media said the security guard, called Eduardo, only realised had been stabbed when he boarded the team bus with the Real Madrid coach.
He thought he had been punched until he saw the blood pouring from the wound.
Real Madrid security chiefs believe the attacker meant to harm Mourinho.
The bodyguard was treated for a 4cm wound.
Mourinho was only told about the incident yesterday, Spanish media said.
Newspaper El Periodico said: "Mourinho stopped to sign autographs in arrivals, where a large number of Madrid fans were waiting for the team.
"The bodyguard was near the manager and noticed he had been stabbed in the armpit.
"The security guard realised he had a 4cm wound caused by a sharp object.
"Mourinho was very worried by the incident."
Security cameras recorded the drama and the attacker, said to be a tall man, has been identified.