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Having represented the Czech Republic at every youth level from Under-17 to Under-21, Kalas also has a number of caps for the senior side, making his debut in November 2012 against local rivals Slovakia.
Agreeing a 93-day loan move to Boro in January 2015, Kalas hit the ground running on Teesside and made 17 appearances - primarily at right-back.
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After two seasons on loan at Dutch outfit Vitesse Arnhem, during which time he was named Czech Republic Young Player of the Year, he made his debut for his parent club in a 2-0 League Cup win at Arsenal in October 2013.
"Slavisa (Jokanovic) was calling me a few times and he was trying to tell me that they really wanted me. That's one thing that made me decide to join Fulham."
An impressive display at both ends of the pitch saw Fulham beat Preston 5-0 on the final day of their Cork trip.
Chelsea defender Tomas Kalas has agreed a season-long loan move to Fulham - ending speculation linking him with a return to Middlesbrough FC.
But he did not stay in west London, instead enduring a difficult loan spell at German side Cologne before Boro came calling.
Fulham made it a hat-trick of signings when they captured Chelsea defender Tomas Kalas on a season-long loan last night.
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He once compared himself to a training cone - such was his lack of playing opportunities with Chelsea. He explained: "I am a player for training sessions. They need a cone, they put me there instead."
The defender wants to stay in England next season :: He's previously said he'd be open to a return to Teesside
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A centre-back who can also operate on the right, Kalas spent the previous 18 months on loan with Middlesbrough, playing 47 times as they reached the Championship Play-Off Final in 2014/15, before winning promotion as runners-up behind Burnley last season.
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The 23-year-old Czech Republic international has spent the last 18 months on loan at Middlesbrough, making 47 appearances.
Watch the pick of the action as Fulham put five past Preston North End in Cork on Tuesday afternoon.
"Fulham were really active trying to call me and I was happy to see that interest and that the club wanted me," Kalas told the club's website.
Chelsea FC Membership for the 2016/17 season offers fantastic choice and a variety of benefits...
"I'm happy to sign because when you see the interest, it's one of the most important things. Middlesbrough was something different and hopefully I'll play here too.
Hospitality Department Chelsea Football Club Stamford Bridge Fulham Road London SW6 1HS
The versatile defender has spent the last 18 months on loan at Boro , making 47 appearances for Aitor Karanka's side.
The Club is delighted to announce the signing of Tomas Kalas on a season long loan deal from Chelsea.
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In July 2013, ex-Chelsea boss, Jose Mourinho, said that Kalas would stay at the club to fight for his place in the team, he made 3 appearances in that pre-season. Unfortunately, he suffered a fractured fibula, cutting his pre-season short.
He went on to make a further 26 league appearances last season.
New signing Tomas Kalas sees no reason why Fulham can't fight for promotion from the Championship in 2016/17.
Kalas, Tomáš - Fulham (ENG)
He was forced to watch Boro’s unsuccessful play-off campaign from the sidelines after his loan ended early, but pledged his short-term future to the club only two months later by inking a season-long loan deal.
Earlier this week the player's agent Mithat Halis claimed Boro were keen on signing Kalas for a third spell this summer.
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The 23-year-old began his career at his hometown club Sigma Olomouc, joining Chelsea for a reported £5.2m in the summer of 2010.
Kalas joined the Blues in 2010 and has featured four times for the Premier League club, while also having loan spells at Sigma Olomouc in his homeland, at Dutch side Vitesse Arnhem and with German outfit Cologne.
The man himself, who can play both centrally and on the right of defence for Whites , reveals all in an interview below. But what else is there about a much travelled player yet to make an impact at Stamford Bridge?
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Czech mate moves down the road from Stamford Bridge via Teesside on loan for Championship 2016-17 and makes bold prediction about Whites chances