58-year-old Eddie McKay was reportedly stabbed twice in the arm and once on the back after being accosted by a group of up to eight individuals wearing balaclavas while with his son on Monday.
Newcastle United begin their first Champions League campaign against AC Milan at the San Siro later on Tuesday.
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."
The victim's daughter, Rachel McKay, told Sky Sports News: "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."