According to Tgcom24 broadcaster, carabinieri were able to apprehend the attacker, a 46-year-old Italian man.
The reason for the attack is yet unclear, but the terror act version was ruled out, according to the broadcaster. Although a motive is uncertain at present, police have suggested the attacker's mental health played a role.
Several ambulances and a special medical helicopter arrived on the scene and transported the victims to hospitals. Three of the injured are said to be in critical condition.
Italian broadcaster Rainews24 later reported that on the way to the hospital, one of the wounded, who worked in the mall as a cashier, died.
A release since issued by the Arsenal club has indicated that that Mari, who is on loan at Monza from Arsenal, did not sustain serious injuries.
"We are all shocked to hear the dreadful news about the stabbing in Italy," the statement read. "Our thoughts are with Pablo and the other victims of this dreadful incident."
It was later detailed by the Monza chief executive Adriano Galliani that the player had a "fairly deep wound on his back" but that it did not puncture any vital organs.
An investigation into the incident remains ongoing.