Chelsea defender Tomas Kalas has been loaned back to Middlesbrough for the 2015-16 season.
The Czech Republic Under-21 international made 17 appearances in three months for the Championship side last term.
His loan spell ended in mid-April, so he could not help Boro during their run to the playoff final, where they were beaten by Norwich City.
"It was a bit disappointing I couldn't play, but this is a new challenge and hopefully we can get promoted without the playoffs," he told the club website.
Kalas, 22, has made just one substitute appearance for Chelsea since joining them four-and-a-half years ago.
