Chelsea defender Tomas Kalas will return to Middlesbrough on a season-long loan spell.
The 22-year-old joined Aitor Karanka's side during last January's transfer window and will feature at the Riverside Stadium next season as well.
"I'd already decided at the end of last season that if Chelsea would like to get me on loan again, this would be my first choice, so it was easy when I heard I could stay," the player told Middlesbrough's official website.
"It's been easy to adapt because I know the lads.
"I know Marbella because we came here last year, so it's been easy.
"I think we're still building the team. It's a mix of everything. Everyone is still working on it and I think we will be stronger than before."