"We must be really pleased with the situation now. He has been a key player for us since he has been here and now we know he can be a key player in our future," said the Reds boss.
"From the start we knew Javier was a player with quality and a fantastic mentality. We knew the player when he was in the reserves at River Plate but was playing for the senior national team and we had been monitoring his career since then.
"He is young but he has great experience having played for his country. We knew he would be a good signing for us.
"It's clear that he is one of the best midfielders in the world but he can get better with us. We now have a lot of good options in the centre of midfield and that's good news for the club."
"Our overriding thought is that we are delighted to have signed a world class player on a long contract and I know Javier is thrilled too because he was keen to have his long term future resolved," added Chief Executive Rick Parry.
"I think this signing demonstrates two things. Firstly, that life at the club goes on despite all of the speculation to the contrary.
"And secondly it demonstrates in very large measure the ongoing commitment of the owners who once again have come good when funds have been needed."