Mari, who is on loan at Italian side Monza, was one of five people stabbed at a shopping centre earlier on Thursday. He was understood to be taken to a hospital in northern Milan.
Police initially said there had been six stabbing victims but later said the sixth person was treated for shock.
An unnamed 46-year-old man reportedly stole a knife inside the shop and “without warning, started hitting random people”, injuring five persons and killing one. He has since been arrested by police and taken into custody.
ANSA news agency reported that a 30-year-old supermarket employee died en route to the hospital.
In an update late on Thursday night, Monza CEO Adriano Galliani said Mari had sustained a deep cut to his back during the harrowing incident but was conscious and talking and may have been saved due to his height.
“Pablo Mari has quite a deep cut to his back, but fortunately it did not reach any vital organs like the lungs or anything else,” he said. “His life is not in danger, he should recover quickly. He has some lesions, but not very serious.
“I have spoken to Pablo, he is conscious. He told me about this incredible incident. He had his son in his trolley and his wife next to him. He did not notice anything. He then felt something painful in his back, which was the criminal’s knife. I imagine he was saved by his height because he was struck on the shoulder.
“Unfortunately he also saw the criminal stab someone in the throat. He saw everything that happened and it was deeply disturbing.”
Asked how Mari is doing mentally, Galliani said: “I thought he was doing pretty well but I’m not a doctor so I can’t say much. He’s clear-headed. Pablo is strong! He said he will be playing on Monday evening! He has incredible strength.”
Gunners manager Mikel Arteta said upon hearing the news: “I know [technical director] Edu has been in touch with his relatives. He is in hospital. He seems to be OK. I will get a briefing now about what happened.”
Mari is on loan at Serie A side Monza from Arsenal for the season. The deal includes a buy option, which would become permanent should Monza avoid relegation, which they are expected to do.
Arsenal are in contact with Monza and will continue to be so as the situation develops over the coming days.
“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,” said a statement from the club.
“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.”
Galliani added on Twitter: “Dear Pablo, we are all here close to you and your family, we love you, keep fighting as you know how to do, you are a warrior and you will get well soon.”