Kalas re-joined Middlesbrough on a season-long loan last week after making 17 league appearances for the Championship side last season.
The 22-year-old defender moved to Chelsea in January 2011 from home town club Sigma Olomouc in the Czech Republic and has also spent time on loan at Vitesse Arnhem and FC Cologne.
Kalas was handed his Chelsea league debut in April 2014 but has made only four first team appearances for the Blues.
Source : PA
Source: PA