"He is a player with character and personality. He is also a holding midfielder we can use if we have problems," said Benitez.
"He has experience and the quality we need. We know he was not playing well at West Ham but I know the qualities he has.
"He is a player for the future also, and can give us character and quality. He will come here for at least 18 months and will put pressure on other players.
"We have seen the player before. He was a player I knew when he was at River Plate. He was in the reserve team at River Plate but was training with the national team when he was 18, which shows the quality and character of the player.
"I had a meeting here with one of his former managers and before he knew we were interested in signing him he was talking really well about him as a player, as a person and as a professional.
"In the World Cup he played really well. Some people say we don't need any more midfielders, but we only have Xabi Alonso who is a holding midfielder."