The 25-year-old had only made three substitute appearances for Ronald Koeman’s side in the Premier League this season and he spent most of last season on loan at Hull City.
The Uruguay international joined Southampton from Bologna in a deal reportedly worth £12 million in 2012 and will now continue his career in the Championship with Middlesbrough.
Gaston Ramirez, who has 34 caps for his country, joins a team in a strong position to win promotion this season. They are second in the table with 55 points, one adrift of leaders Hull with a game in hand.
By Taiwo Oyedeji
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