Barcelona confirmed on Tuesday they have struck an agreement for Martino to take over at the Nou Camp on a two-year deal.
The 50-year-old Argentine succeeds Tito Vilanova, who stepped down from the role on Friday after one season in charge as he continues his battle with cancer.
It is not yet known when Martino will begin work at the Nou Camp, but it seems likely the transfer saga surrounding Fabregas will still be rumbling on with United reluctant to throw in the towel.
United have already made two bids for the former Arsenal captain but, with Barcelona continuing to insist the Spain midfielder is not for sale, the Red Devils could return with a third offer.
Fabregas was not on the agenda on Tuesday night as Martino held a news conference in Rosario where he admitted Barcelona's interest had come out of the blue.
The former Paraguay manager also believes fellow Argentine Lionel Messi had a say in his appointment.
Both Martino and Messi are from the Rosario region of Argentina and the four-time Ballon d'Or winner has spoken in glowing terms about his compatriot in the past.
Martino suspects Messi put in a good word for him with the Barcelona hierarchy.
"I don't know the details of the situation, [but] I have no doubts that Messi spoke with the club directors. I'm sure that Jorge [Messi's father] and Leo have had some weight in this decision," said Martino, who is looking forward to working with the world's top player.
"As well as the responsibility of being the coach of Barcelona, it's obviously a huge responsibility to coach the best player in the world. We will try to find the right formula so that we can help him continue to be the extraordinary player he already is."
A host of managers were linked to the position at Barcelona, including Luis Enrique, Andre Villas-Boas, Marcelo Bielsa, Michael Laudrup and Guus Hiddink.
However, Martino emerged as the favourite over the weekend and the former Argentina midfielder said: "Of course it took me by surprise.
"I had virtually prepared myself to take a break and wait for the right job opportunity to arise.
"It's a big challenge to join a club of this magnitude and to try to do a good job. As with every big club, I want our team to be serious candidates to win every competition."
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