Kalas, now 21, was signed as an 18-year-old back in 2011, with the Blues paying over £5m to sign the defender from Czech side Signa Olomouc. With Chelsea's recent transfer policy of loaning out young players to get first team experience, it was no surprise to see Kalas sent out to feeder club Vitesse for two seasons, where he featured regularly.
Upon returning to Stamford Bridge, Kalas found it tough to break into the first team as the likes of Branislav Ivanovic, John Terry, David Luiz, Gary Cahill and Cesar Azpilicueta all took positions in Jose Mourinho's side, leaving Kalas on the fringes in West London.
Yet in April, Kalas was finally handed his first Premier League start in Chelsea's big game at Anfield in place of the injured John Terry, and the Czech international impressed massively, exciting Blues fans in the process as the hoped the youngster would continue to develop into a key starter under Mourinho.
However, Kalas – who can play at both right back and centre back – appears set to spend another season away from Stamford Bridge after it was today revealed that the 21-year-old is once again on the verge of a season-long move, this time to Germany with newly promoted Koln.
With Terry's career coming towards the end and David Luiz poised to finalise a blockbuster switch to France with Paris Saint-Germain, Mourinho needs to start planning for the future in the heart of his defence. St. Etienne defender Kurt Zouma should play a role next season, but now with Kalas out gaining experince, there seems to be a clear plan for the years ahead.
Playing in a high standard league like the Bundesliga will be great for Kalas' development, and much more beneficial than playing in the Eredivisie or for Chelsea's reserve side. If he can impress with Koln then he can come back next summer and once again push for a place in the Chelsea first team – yet despite Kevin de Bruyne impressing in Germany with Werder Bremen, it still wasn't enough to convince Mourinho that he had a future with the club – meaning Kalas' future could be determined by the success of this loan spell.