The 21-year-old will spend his third season on loan from Chelsea after playing for Dutch top-flight side Vitesse Arnhem in 2011-2013.

"In the past two years, Tomas's performance has improved so much that to develop his skill and gain experience, he could only join a Premiership or Bundesliga side," said Kolar.

Kalas, who played one game for the Czech national team in 2012 and appeared in only a handful of games for Chelsea in the 2013/2014 season, said he was looking forward to the new experience.

"I'd like to play regularly to return to Chelsea even stronger. The Bundesliga in a famous team like this will be a great test for me," he said.