The Monza defender was one of six victims involved in a knife attack in a supermarket which left one person dead.
Mari was with his wife and son when the incident happened.
The 29-year-old is on loan at the Serie A side from Arsenal and has made eight Serie A appearances this season, scoring one goal.
A 47-year-old Italian man has been arrested by police as the culprit.
"I fear it'll be three months before he can be back on the pitch as he can start training after two and put in a big effort as he makes progress," Monza chief executive Andrea Galliani told Sky Italy.
"In sporting terms, unfortunately it'll be more than two months until he's back on the pitch.
"What I've understood from the doctor is that if he hadn't had strong muscles in his back, the knife probably would've gone in more deeply and he would've died. Being an athlete certainly helped him in terms of having strong muscles that stopped the knife."
Quoted in Gazzetta dello Sport, Mari said: "I was lucky, because I saw a person die in front of me."
He later said in an Instagram post: "After the hard moment we experienced yesterday, both my family and I want to communicate that fortunately we are all fine despite the circumstances, and we want to thank all the messages of support and affection that we are receiving.
"In addition, we want to send our condolences and all our strength to the family and friends of the deceased person, and we sincerely wish that all injured people recover as soon as possible."
