The resignation of Arsenal's sporting director Edu on Monday appeared to catch everyone at the club and in the wider game by surprise.

As recently as mid-September, when Arsenal announced a new long-term contract for head coach Mikel Arteta, Edu went on record about his "strong belief in what we want to achieve together" and, as one of the 'Invincibles', he felt far more emotionally-invested in the club than your average ambitious executive.

He is, however, set for a new and - if reports that he will more than double his salary are accurate - enormously lucrative role in charge of the network of clubs owned by Greek magnate Evangelos Marinakis, which include Nottingham Forest.

Edu's departure is hardly a catastrophe - there tends to be a high turnover of sporting director-types at many leading clubs - but it has to go down as a major hiccup in Arsenal's otherwise seamless upward trajectory over the last three years or so.

Like any high-profile figure in the elite game, Edu has occasionally divided opinion but it is hard to seriously make a case that he has done anything but an outstanding job since returning to north London as technical director in July 2019.

Back then, the idea of Arsenal challenging for the title was still vaguely ridiculous - they had just finished fifth, 28 points behind Manchester City after a limp end to the domestic season which felt wholly characteristic - and yet today, one bad result verges on a crisis for Arteta's side, who have twice pushed the champions close.

Practically every one of Arteta's most trusted players, with the notable exception of academy graduate Bukayo Saka, have been signed on Edu's watch, and he has an excellent relationship with the head coach, owners, players, families, agents and the media, as well as a wide remit which also covers the academy and the women's team.

His partnership with Arteta, in particular, has been a resounding success, the pair always seeming to be smoothly aligned on transfer policy and vision.

Edu's record of offloading unwanted players has been mixed but Arsenal were better on that front last summer, landing decent fees for homegrown pair Eddie Nketiah and Emile Smith Rowe, as well as No2 goalkeeper Aaron Ramsdale.

Arsenal have given every impression of being one of England's best-run clubs in the past half-decade but Edu's exit will now test this hypothesis.

The turbulence of the post-Arsene Wenger era was only deepened by a revolving door of executives - Raul Sanllehi and Sven Mislintat were among Edu's short-lived predecessors - and Arsenal are in danger of being destabilised again if they do not move quickly to replace Edu, and with the right person.

The club may need a big January if City and Liverpool maintain the gap at the top, while they are still short a marquee No9 and face contract negotiations with Saka and William Saliba in the near future.

Edu's exit also comes at a delicate time for the Gunners, who are winless in three Premier League games ahead of Sunday's visit to in-form Chelsea.

Last weekend they were bailed out by Bournemouth, whose shock win over City ensured the champions did not open up an eight-point gap over Arsenal, which might have felt borderline insurmountable given the standards set by Pep Guardiola's side and their tendency to ominously hit their stride after Christmas.

Arsenal can scarcely afford to keep throwing points away and a messy Edu succession may prove an unwelcome distraction from the first-team, not least for Arteta.

Unsurprisingly, City again feel like a relevant benchmark for Arsenal.

Within days of the news that the champions' director of football Txiki Begiristain would retire at the end of the season, City had announced his successor in Sporting Lisbon's Hugo Viana, who will shadow Begiristain in the months ahead to ensure a smooth transition.

The two situations are not the same, but City's ability to seamlessly transition between eras is part of what makes them such a formidable opponent.

Arsenal's challenge is now to enact a similarly efficient succession for Edu to ensure his exit does not derail the club's progress.