Middlesbrough FC: 2015 Chelsea deal handed Aitor Karanka with unsung Premier League promotion hero | OneFootball

Tomas Kalas enjoyed two loan spells on Teesside, and played a huge, yet sometimes forgotten role in Boro's promotion to the Premier League.

Middlesbrough manager Aitor Karanka was in the midst of building a team that was ready to win Premier League promotion come the 2015 January transfer window.

Having become Boro's first ever foreign manager after replacing Tony Mowbray in 2013, the former assistant to José Mourinho at Real Madrid was starting to turn the Teessiders into a real force to be reckoned with in the Championship.

Middlesbrough were becoming tougher and more resolute in defence, whilst their cutting edge in attack was being sharpened with some highly shrewd additions.

Optimism was returning to the Riverside Stadium that top flight football was on the horizon once more, but in order to get there, Karanka would need a strong and deep squad brimming with quality options.

He'd find just that sort of player in the winter window of 2015, after striking a deal with Premier League heavyweights Chelsea.

Middlesbrough secure short-term loan deal for Chelsea defender Tomas Kalas

Signed on an initial short-term loan deal, Tomas Kalas provided Karanka with a much-needed body in defence, as Boro looked to kick on towards cementing a play-off place in the 2014/15 season.

Just 21 at the time, the Czech Republic international was a young defender who hadn't been handed many first team opportunities at Stamford Bridge since signing for Chelsea in 2011, but had enjoyed a highly fruitful two-year spell on loan in the Eredivisie with Vitesse between 2011-2013.

Having made over 70 appearances for the Dutch outfit, Kalas had proven himself to be ready for senior football at a good standard, and with competition for first team places fierce at Chelsea, he was handed the chance to play regular football with Middlesbrough in 2015.

The Czech would make 17 appearances for Boro in the second half of the 2014/15 season, starting all but one of those games. It was during this spell that he highlighted his qualities as well as his versatility, alternating between centre-back and right-back.

It was this attribute that proved to be worth its weight in gold to Karanka, as his Middlesbrough side went all the way to the Championship play-off final that year, before falling to a 2-0 defeat to Norwich City in the final at Wembley Stadium.

He played an understated role in Premier League promotion

Kalas had become a popular figure among the Riverside faithful for his battling displays and his ability to play numerous roles in the squad, and so, despite returning to Chelsea at the end of the season, Karanka knew that he wanted him back in red and white for the following campaign.

Luckily for Middlesbrough, so did Kalas, as he said in an interview with the club's website via BBC Sport in the summer of 2015: "I'd already decided at the end of last season if Chelsea would like to get me out on loan, this would be my first choice."

To the delight of every Teessider, it was announced that Kalas would be returning to the club on a season-long loan deal for the 2015/16 season, and what a year it was going to be for Middlesbrough.

Boro went on a rollercoaster of a campaign, with dramatic highs of multiple last-minute winners, to the lows of a brief Karanka walkout in March, to secure automatic promotion to the Premier League with a 1-1 draw vs Brighton on the final day of a memorable season.

Whilst other star players - and rightly so - grabbed the headlines and became the poster boys of that Karanka era, the impact and importance that Kalas had on helping the club win promotion that season can sometimes be forgotten.

Like he showcased in his first loan spell, his versatile nature enabled him to plug gaps and fill in both in the centre of defence and at right-back, making 26 appearances in the Championship that term.

Boro's defence conceded just 31 times in the league during the 2015/16 season, the fewest number of any side in English football's top four divisions that year. They would also set a new club record for the most consecutive home clean sheets with nine.

Breaking those records and keeping such a watertight defence wouldn't have been possible if Karanka couldn't turn to such a reliable option such as Kalas, with the Czech ensuring no harm would come to Middlesbrough if and when he was called upon.

Kalas was as tough and as solid as they came, instantly slotting into the dynamic and the chemistry of that outstanding Middlesbrough squad. So, when Boro fans think back to those days and that squad in particular, Kalas should always be right there among the household names and cult heroes.