Aitor Karanka has welcomed an "experienced and committed" player to the club, following the capture of Southampton's Gaston Ramirez.
The Uruguayan international, who cost the Saints £12m from Bologna in 2012 and is experienced in the Premier League, has joined the club on loan until the end of the season.
"I am really pleased," said Boro's Head Coach.
"He is a really good player who knows the league and the country.
"He has international experience and has played in the World Cup. He is another good player for us.
"He is the kind of player who wants to join us and that shows he is committed to us. He has played in the Premier League and I think it is important to bring in players with this kind of experience."