Eddie McKay, 58, from Newcastle, was with his son and a friend when they were attacked by a group of seven or eight people, who he claims were wearing balaclavas.
He is in a stable condition at hospital, after being stabbed around midnight last night.
A Newcastle statement said: “We are deeply concerned by reports that a supporter was seriously assaulted in Milan on Monday evening and we are liaising with local authorities to understand the circumstances.
“Our thoughts are with the supporter and their family and we hope for a full and speedy recovery.”
Mr McKay received stab wounds to his back and arms after he fell to the ground while trying to run away.
Police in Milan confirmed Mr McKay was treated for injuries by paramedics at the scene and is in a city hospital.
His family are flying to Milan later on Tuesday to be by his side.
The victim’s daughter, Rachel McKay, “My dad is stable and in a bit of shock.
“He’s gutted he’s missing the match but just wants every other Newcastle fan out here to be aware and be safe.
“He has just had two knee replacements so he fell when he was being chased and that is when he was attacked.
“My brother is okay but is in shock after seeing that happen to our dad. I’m flying out there later. We haven’t slept all night.”
Police say they don’t know if the attack was for football reasons or other reasons.
Newcastle play their first Champions League match for 20 years on Tuesday night against AC Milan in the San Siro.
The club have 4,000 tickets for the fixture, but further ticket less fans have traveled to Milan ahead of the game.