Kalas, who joined Middlesbrough on loan last week, signed for the Blues in January 2011 from home town club Sigma Olomouc in Czech Republic.

After a successful loan spell with Vitesse Arnhem in the Netherlands, he became a part of Jose Mourinho’s first team squad for the 2013/14 season, making his debut as a substitute in a 2-0 League Cup victory away at Arsenal.

Kalas made his full debut in April 2014 in another impressive away win, the memorable 2-0 victory over Liverpool at Anfield, where he impressed at the heart of defence alongside Branislav Ivanovic.

Kalas was a part of the Czech Under-21 squad for this summer’s European Championships, and will now spend the 2015/16 season back with Middlesbrough.