The 21-year-old spent the first half of the 2014-15 campaign on loan with Cologne but failed to make a single league appearances for the Bundesliga outfit.
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Chelsea boss Jose Mourinho has subsequently decided to send Kalas, one of the Blues’ most highly-rated young defenders, to continue his development under Aitor Karanka, who worked as Mourinho’s assistant at Real Madrid.
Kalas joins Chelsea starlets at Boro
Kalas joins fellow Chelsea youngsters Kenneth Omeruo and Patrick Bamford at the Riverside Stadium and will be expected to help Boro secure a place in the play-offs.
Automatic promotion remains a real possibility, however. Middlesbrough are currently fourth in the Championship table, five points behind league-leaders Bournemouth, midway through the campaign.
Kalas showcased talent last season
Kalas made two Premier League appearances for Chelsea last season, and showcased his immense potential in the Blues’ memorable 2-0 win over Liverpool at Anfield in April.
The Czech Republic international replaced the injured John Terry and played the entire 90 minutes as Chelsea dented Liverpool’s hopes of winning their first league title since 1990.
A month earlier Kalas made his Champions League debut, replacing Willian in the last 16 clash against Galatasaray.
Battle for first-team place
Despite his undoubted quality, the centre-back faces an uphill struggle to establish himself as one of Chelsea’s first-choice defenders.
Although Terry is approaching the winter of his career, Chelsea have a ready-made replacement for the Blues’ long-serving captain in the form of France Under-21 international Kurt Zouma, who has made 10 appearances for the west London outfit this term.
Kalas will at least have the opportunity to show Mourinho why he deserves to be in contention to become a first-team regular for the 2015-16 season.
An impressive showing at Middlesbrough - culminating in the club’s return to the Premier League - may encourage Mourinho to use Kalas in pre-season when he returns to the club this summer.
But Chelsea are a club constantly on the lookout for ready-made stars - and, as one of the world’s most wealthiest clubs, will not hesitate to sign a top centre-back if the right player becomes available later this year.