Mental toughness is a difficult attribute to explain. It is an even more difficult attribute to quantify.

Whatever it is, Diego Simeone treasures it in his players and that steel and determination that has come to define his Atletico Madrid team is present in all of his key players.

For Thomas Partey, an under-the-radar midfield general who doubled up as an emergency full-back in last week’s game at the Emirates, it comes from as difficult a route into football as one could find, the polar opposite of the cosseted upbringings enjoyed by academy graduates from the Premier League’s elite.

The midfielder has become one of Atletico's key players (Getty)

“After every game, I speak to my father,” Partey tells The Independent. “He is my number one fan and I will never forget how we were and when it all started for me.

“We have come a long way. You have seen my entire family back in Ghana and I am conscious to how we use to live so yeah some people forget where they come from but I have a mental picture of my father so I keep it as a reminder on my way to the top.”

In 2011, Partey’s father was faced with a choice. His teenage son had been spotted playing football by an agent who believed he could take him to Spain and earn him a professional contract. The Partey family did not have the money required but, unbeknownst to Thomas, his father sold off some property to fund the move.

There was a player better than him, a prolific striker, but he had neither the financial backing nor the inclination to uproot his life and move several thousand miles without the promise of so much as a single euro.

He has been a vital player for Atletico this season (Getty)

“I got into a car, they took me to the capital, they gave me my passport and said: 'today you travel.’”, Partey recalled earlier this year in an interview in Spain.

“My dad wasn't at home, nobody from my family knew anything, nor that I was going that day because if they were told then it would cause a lot of problems.

“I travelled to Spain and it was six or seven months before anyone realised that I wasn't in Ghana.”

What followed is a fairytale for the boy from Krobo Odomasi, a contract with Atletico Madrid’s reserves blossoming into loan moves at Mallorca and Almeria that were specifically picked out to aid the young midfielder’s development.

A first-team debut followed in 2015. The next season he would shine in a dual assault on La Liga and the Champions League and it was a campaign from which he never looked back.

“When I first arrived here I didn’t know anyone and adaptation was very difficult but the technical team and the coach from the first day were very supportive,” He told The Independent this week.

“Then there was that breakthrough season in 2016 after the loans and now I am here. What can I say but it’s been living my dream and playing for a club i fell in love with from the first day I arrived here.

Partey was superb against Arsenal (Getty)

Partey has consistently been one of the best players for Atleti this season but his performance at the Emirates in the first leg of the Europa League semi-final, when Sime Vrsaljko had been sent off inside the first 10 minutes, proved why his coach Simeone refers to him as the unsung hero of his team.

The Ghanaian slotted straight into the right-back position and alternated between playing at right back, defensive midfield and even attacking midfield as Atletico fought back to take a 1-1 scoreline - and crucially an away goal - back to Spain despite playing 80 minutes a man down.

As a true Simeone disciple, however, Thomas is never happy if it’s not a win or a clean sheet.

Diego Simeone has played a huge role in his career (AFP)

“Firstly, for me the sending off was a wrong call from the referee and sometimes its easy for us to just dwell on it and lose the game but we were so determined to get something from there.

“The thing about Simeone a lot of people don’t know is that his instincts are always right, we trained playing 10 vs 11 the day before the game and he kept telling us that we might end up a man down and it came earlier than expected.”

“I feel so privileged being trained by such a coach for some time now the media always talks about how well players are paid and how we live a luxurious life but for those who know me I keep saying that every player will improve like me if they train under the boss.

“I don’t know what other clubs do but every training session is a battle here because the team keeps changing based on how hard you train and perform in games.”

Partey on international duty with Ghana (Getty)

That hard training has paid off for Partey, whose role in Atletico’s success continues to grow with each season that passes.

Now he’s targeting Europa League success, but first he knows they must finish off Arsenal at the Wanda Metropolitano.

“The tie is not over but of course not to sound arrogant we know what we have to do to get to the final.

“They have their mission to sign off Wenger very well but we also have to continue to ensure that every season we are among the best in Europe and that is where our fire comes from.

“Things can change so fast in football so we don’t have to lower our guard we know we have world class players but Arsenal has also world class players so it will all come down to the determined team so we will be ready for them for another battle.”

With Arsenal in mind, Partey has attracted plenty of Premier League interest. But it’s nothing that’s going to distract him as long as he has the faith of his manager.

Unsurprisingly, there has been Premier League interest (Getty)

“I don’t really read too much into what the media say regarding me moving, I leave that all to my management but of course everybody knows the English game is one of the best in the World.

“I have played in their stadiums twice already this season at Stamford Bridge and at the Emirates and I know how the atmosphere is but at the minute I am so happy in Madrid.