The ex-Argentina international spent eight years at St James' Park having joined from Deportivo La Coruna in 2008, helping the team into the Premier League in 2010 and making 275 appearances for the club.
"I was immersed in a dream for 22 years of my life that has given me much more than I imagined," he wrote on Instagram.
"I fulfilled all my dreams as a soccer player, (and) know that I was happy during this trip which I would have liked never to end.
"But unfortunately it has an expiration date. Like everything."
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