This evening, Belgian goalkeeper Thibaut Courtois will play for his current loan team Atletico Madrid against parent side Chelsea in the semi-finals of the Champions League.

Legally, there is no clause that prevents the shot-stopper from facing the club to which he belongs, but it is understood that Chelsea were keen for the 21-year-old not to be selected.

At the very least, the Blues reportedly wanted a substantial figure from Atletico for the privilege, but UEFA intervened and declared that no such sum would be enforceable.

Here, Sports Mole discusses just how important Courtois has been to Atletico's success this season and why Chelsea would be wrong to let him leave in the summer.

Had Courtois not been a professional footballer, he could have represented his country at beach volleyball. At least that is the view of those who knew him from an early age, when his family home had a beach volleyball court in its garden. It was through that particular sport, according to his mother Gitte Lambrechts, that the Belgian was able to hone his skills from a youth.

"Everybody who does volleyball at a high level will spot many things they recognise when they see Thibaut play football,' said Gitte in an interview that was quoted by the Daily Mail.

The imposing goalkeeper started his football career with local side Blizen VV as a left-back at the tender age of five, but was converted into a goalkeeper just two years later after joining professional outfit Racing Genk.

It did not take too long for Courtois to make his impression at Genk, with his performances throughout their youth system earning him his first-team debut at the age of just 16. In the 2010-11 season, Courtois received the Goalkeeper of the Year Award after conceding just 32 times in 40 league games as Genk picked up the league title in Belgium.

Those who knew the game were all too aware of his talent and it was Chelsea that secured the teenager's signature on a five-year deal in 2011. Within weeks of moving to Stamford Bridge, however, Courtois was sent on loan to Atletico, who were on the lookout for a new goalkeeper after losing a certain Spaniard, David de Gea, to Manchester United.

In his first season in Spain, Courtois helped Atletico claim the Europa League title. Such success brought an extension to that loan and it proved to be another impressive season for the 6'6" keeper, with Atletico finishing third in La Liga in addition to winning the Copa del Rey with a 2-1 win over Real Madrid. In the league last season, Courtois kept 20 clean sheets as Atletico conceded just 31 times, which was the strongest record in the division.

As it stands, Courtois is in his third season on loan with Atletico and it promises to be his most successful. Atletico currently lead La Liga by four points with just four games remaining as they chase their first league title since 1995, while they, as mentioned, will face Chelsea in the semi-finals of the Champions League. Courtois has already kept 19 clean sheets in the league this season, while Atletico have conceded just 22 times overall.

All of this from an individual that is yet to make his first-team debut for Chelsea and one that is far from certain of a first-team spot at Stamford Bridge next season. Petr Cech has been a brilliant servant for the Premier League club, but it is difficult to argue that Courtois is not now the better bet for Jose Mourinho, who is fully aware of Courtois's talents following his time at Real Madrid.

It is little secret that Real Madrid are also interested in acquiring Courtois – they will have to join the queue - while Barcelona hold a firm interest despite a move to the Camp Nou being unlikely. Courtois might be just 21, but for many, he is already the best goalkeeper in the world.

In truth, it is difficult to spot a weakness for Courtois. Nobody is perfect, but the Belgian number one is aerially dominant, has incredible shot-stopping reflexes and his size makes him so strong in one-on-one situations. Add into that Courtois's stunning distribution and mental toughness and it is not too difficult to see why there is so much fuss surrounding his inclusion in tonight's clash at the Calderon and indeed his long-term future.

While Atletico's success this season can be put down to a number of factors - including the tactical genius of head coach Diego Simeone, the goals of Diego Costa, the work-rate of Gabi and Arda Turan and a formidable defensive unit that includes Juanfran, Diego Godin, Miranda and Filipe Luis - it is difficult to downplay just how important Courtois has been to the Spanish side this season.

As it stands, Courtois is due to return to Chelsea at the end of the season. However, it is understood that the imposing goalkeeper wants assurances over first-team football before he returns. To put it bluntly, Courtois is not and cannot be a number two. He cannot sit on the bench next season. Not at Chelsea, not anywhere, which is why another loan move to Atletico has been mooted.

A final decision is expected to be made before the World Cup this summer, where Courtois will travel as Belgium's number one goalkeeper. He might only just be out of his teens, but Courtois played every minute of his country's 2014 World Cup qualification campaign and has already turned out 14 times for a nation that are many people's outside tip for the tournament in Brazil.

"He was a guy whether he was playing in front of 30,000 or in his back garden. Playing against a team in the fifth league or the Europa League was the same for him," said head of goalkeeping at Genk, Guy Martens, in an interview also quoted by the Daily Mail.

It is that innocence and lack of fear that has elevated Courtois's progress in Spain to a level that few expected at this early stage of his career. Little under 22 years ago, Thibaut Nicolas Marc Courtois was born in Bree, Belgium, just 12 minutes from the Cristal Arena Stadium, where Genk play their home matches. Now, he is the most talked about goalkeeper in world football.