The Argentina international striker's return to his homeland was finalised last night, with Black Cats boss Gus Poyet confirming the deal following Sunderland's 3-0 pre-season win over Hartlepool United at Victoria Park.
Poyet said: "We were close on Nacho's deal before this game [at Hartlepool] and I've just been told it's all been signed.
"He wanted to go back to Argentina, it was a good deal for us.
"It has been the finer details we have been waiting on.
"He hasn't played because it looked like he would move on.
"It is all signed now with Newell's Old Boys."
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