His relationship with Txiki Begiristain -- who he worked with at Barca -- made Manchester City the obvious choice, but Sanllehi will become Arsenal's new head of transfer operations and will work with former Borussia Dortmund chief scout Sven Mislintat, who joined the club earlier this month to head up recruitment operations.

So who is Sanllehi and what are Arsenal getting when he starts work in February?

1. Staying power shows importance

After studying in the United States -- at Guilford College in North Carolina -- Sanllehi worked various jobs before landing a role at Nike in 1996. That brought him into contact with Barcelona and in 2007, having worked at the club since 2003, he was eventually appointed Barca's Director of Relations with Sports Bodies. Within a year, he was handed the Director of Football role, which he's held for the last decade.

What stands out is that he's served three presidents in that time: Joan Laporta, Sandro Rosell and Josep Maria Bartomeu. Directors are often collateral damage when a new president comes in, looking to make their mark at the club. Yet Sanllehi's importance is highlighted in the fact he's been able to transcend the Laporta-Rosell quarrels. That is no mean feat and would suggest his work is valued, by those who matter, extremely highly.

2. Neymar deal

During Barca's tour of the United States in the summer, Sanllehi was flown in for one reason: to try to convince Neymar not to leave the club.

Along with Andre Cury, Barca's representative in South America, Sanllehi played a huge role in persuading Neymar and his father to join the club in 2013, snubbing Real Madrid in the process. He was also helpful in Neymar's subsequent contract renewal in Catalonia.

So when Neymar looked bound for Paris Saint-Germain last summer, Sanllehi was Barca's last card. He held various meetings with the player's father, but was unable to pull one last ace from his sleeve.

3. Works in the shadows

Sanllehi's involvement in signing Neymar is the deal which stands out, but he has played a part in a huge number of signings since he joined the club -- his contacts, professionalism and charisma making him a relatable figure for high-ranking club executives, agents, players and players' families alike.

Albert Masnou, the director of Catalan newspaper Diario Sport, says he's "the best in the business at what he does" and that with his departure, Barca will lose a lot.

"In reality, he's worked in the shadows at Barca, almost unknown, but his work has been excellent," Masnou told ESPN FC.

In addition to the Neymar transfer, Sanllehi led talks with Liverpool's Ian Ayre to bring Luis Suarez to the club in 2014 and also helped bring in Ivan Rakitic and Marc-Andre ter Stegen in the same summer for a combined total of €30 million. Just last summer, he was the man sent to Lisbon to close a deal for Benfica's Nelson Semedo.

At Arsenal, it's likely that Mislintat will identify the targets and Sanllehi, in the shadows, will be left to do the rest.

4. Time for a fresh start

Given how well he is thought of, the obvious question is: why are Barca allowing him to leave?

The official answer is both sides feel, after a 15-year relationship, it's time for a change. Sanllehi wants to take on a new challenge and president Bartomeu wasn't against letting him seek one, given it made his revamp of Barca's sporting department slightly easier earlier this year.

"Life goes in cycles and mine at Barcelona came to an end with the new structure of the club behind the scenes," Sanllehi told ESPN FC in October.

"We analysed it objectively at the club and the conclusion was that my profile didn't fit in, which is perfectly respectable. And I say that without any acrimony -- the opposite, in fact. I say it with a lot of affection for the club. I have been, I am and I always will be a Barca fan and when they play I will still say 'we'. Always."

However, unofficially, there were a couple of trigger points in the summer. One on each side. Sanllehi was disappointed to see the club pull the deal for Nice midfielder Jean Michael Seri at the last moment, a deal which he had negotiated. As for the club, they felt that Sanllehi and Cury missed a trick by not being better informed on Neymar's plans to push through a move to PSG.

5. Work and contacts extend beyond Barca

One of the biggest reasons Barca are lamenting Sanllehi's departure is the contacts book he takes with him. Masnou says it's almost unrivalled in European football.

A large reason for that is his personable nature, but his role, until recently, as one of three directors of the European Club Association (ECA) has also helped him meet a wide range of people.

And his work with the ECA has also had consequences for all of Europe's clubs. He played a major part in FIFA's introduction of the Club Protection Programme, which sees clubs compensated for injuries to their players while on international duty.