Chelsea defender Tomas Kalas has joined Middlesbrough on a season-long loan deal.
The Czech international, 22, spent the second-half of last season on loan at the Riverside, having been in Germany with FC Cologne until January.
Kalas signed for Chelsea for £5.2 million from Czech side Sigma Olomouc in July 2010 but has only played once in the league, a 2-0 win at Anfield against Liverpool in the closing stages of the 2013-2014 season.
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The majority of his Chelsea career has been spent on loan at various clubs. After signing for the Blues he remained at his hometown club Sigma Olomouc before joining Dutch club Vitesse Arnhem for the following two seasons making 76 appearances.
In Jose Mourinho's first season back at Stamford Bridge, he said Kalas would be involved in the squad and told to fight for his place.
After three appearances (one league, one league cup and one Champions League), Kalas was sent to the Bundesliga with FC Cologne at the start of the 2014-2015 campaign until his return to England with Boro.
Last season, he was joined at the Riverside by fellow Chelsea players Patrick Bamford, Kenneth Omeruo, and goalkeeper Jamal Blackman.
Josh McEachran, who signed for Brentford on a four-year deal last week, was also previously on loan with the Teessiders.
The Czech centre-back, who can also play as a right-back could be joined by Dutch defender Nathan Ake.
Jose Mourinho looks like he could give his youngsters more playing time under the guidance of former Real Madrid coach Aitor Karanka, who worked with the Portuguese during his time at the Bernabeu.
Kalas featured for hosts Czech Republic in the UEFA European Under-21 Championships earlier this summer and has represented his country at all age levels.
He made his senior international debut in November 2012, coming on as a late substitute in a 3-0 friendly win over Slovakia, his only senior cap to date.
Championship Play-Off runners-up Middlesbrough begin their season away at newcomers Preston North End on August 9th as they look to secure promotion to the Premier League after a six-year absence.