Currently on a season-long loan at Monza, Mari, 29, is recovering in hospital, with local reports suggesting one of six victims had died.
The BBC said a suspect, 46, has been detained and that the motive for the attack was not clear.
Arsenal gave an update on Mari's condition shortly after their 2-0 Europa League defeat at PSV Eindhoven.
“We are all shocked to hear the dreadful news about the stabbing in Italy, which has put a number of people in hospital including our on-loan centre-back Pablo Mari,” it said.
“We have been in contact with Pablo’s agent, who has told us he’s in hospital and is not seriously hurt.
“Our thoughts are with Pablo and the other victims of this dreadful incident.”
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Arsenal manager Mikel Arteta, said after the match: “I just found out. I know [technical director] Edu has been in touch with his relatives. He’s in hospital but he seems to be OK.”
Monza’s president Adriano Galliani said on the club’s official Twitter account: “Dear Pablo, we are all here close to you and your family.
"We love you, keep fighting as you know how to do. Yyou are a warrior and you will get well soon.”