After a turbulent summer, Barcelona are confronted by several serious doubts as they return to La Liga action on Sunday at home to Villarreal – and visiting coach Unai Emery will be looking to make a major impact with his new team.

Can Koeman convince Messi?

It’s impossible to begin an analysis of Barca’s plight with any subject other than Lionel Messi, following the captain’s aborted attempt to leave and his subsequent outbursts against unpopular president Josep Maria Bartomeu.

Messi made it clear on Friday that he is still very unhappy with the club, making a scathing point in his farewell message to departing team-mate Luis Suarez, who has joined Atletico Madrid.

“You deserved a farewell fitting who you are: one of the most important players in the history of the club. Not to be kicked out like you were. But the truth is by now, nothing surprises me,” Messi wrote on Instagram.

For better or worse, incoming coach Ronald Koeman still has no choice but to centre the team’s rebuilding process around the Argentine, whose demeanour and attitude will continue to be endlessly analysed over the coming weeks.

Messi will be free to sign a pre-contract with another club in January, so Koeman has limited time to convince the skipper that he can build a team worth hanging around for.

If the coach doesn’t quickly succeed in improving the captain’s dark mood, it could turn very ugly, very soon.

Who will play up front?

Allowing Suarez to leave was partly symbolic, with Bartomeu pointedly dismantling the ageing team that suffered Champions League humiliation against Bayern Munich.

But it is also pragmatic: the club is broke – hence the acceptance of Wolves’ bid for Nelson Semedo despite Bartomeu previously describing him as non-transferable – and Suarez’s departure takes a significant strain off a crippling wage bill.

It also leaves the question of who will lead the line. There is no lack of attacking quality, with Messi joined by expensive misfits Antoine Griezmann, Philippe Coutinho and Ousmane Dembele, plus talented youngsters Ansu Fati and Trincao.

But other than the injured Martin Braithwaite, there is nobody who could be classified as a centre forward.

Koeman is expected to field Messi as a false nine, but the Argentine will always drop into the same deeper areas that Griezmann and Coutinho also like to occupy – one of the main reasons they have both struggled to shine.

So the coach has the tactical task of devising a system which encourages his gifted forwards to operate collectively rather than individuals, including the challenge of maintaining a penalty box presence from a squad lacking out-and-out strikers.

If Koeman solves those riddles, the results could be spectacular. But can it be done?

Where are the defenders?

The 8-2 embarrassment against Bayern made it pretty clear that Barca have defensive issues, but Koeman has a startling lack of options to resolve them.

Semedo’s departure means the squad’s only right-back is Sergi Roberto, who is really a converted midfielder.

On the other flank, 31-year-old Jordi Alba remains first choice despite an alarming drop in performances over the past two years, while the club’s desire to sell injury-hit Samuel Umtiti would also leave scarce central options.

The club is attempting to recruit, with Ajax’s Sergino Dest, Norwich full-back Max Aarons and Eric Garcia of Manchester City on the wish list. But the dire financial situation outlined above leaves Barca in a weak position, with no guarantees that proven new faces will arrive.

And just to round things off, goalkeeper Marc-Andre ter Stegen, so often the team’s saviour, will be missing through injury until November.

Can Koeman find a collective shape?

As much as the identity or quality of the individual players, the team’s demise has been based on the lack of compactness in their overall shape.

The wide open spaces between attack and defence have often left the midfield overrun and unable to repel rapid counter-attacks. That weakness was most brutally exploited by Bayern but it was also regularly evident in La Liga, where Barca conceded 13 more goals than Real Madrid last season.

Koeman will attempt to instil greater solidity by employing a 4-2-3-1 formation, with fellow Dutchman Frenkie de Jong joined by either Sergio Busquets or new signing Miralem Pjanic in midfield.

But unless the back four pushes higher up the field and the forward line of Messi, Coutinho and company are cajoled into playing their part defensively, it remains doubtful whether a more effective, collective structure can emerge.

Will Bartomeu be forced out?

Overshadowing everything behind the scenes is the fate of beleaguered president Bartomeu, who faces a vote of no confidence after more than 20,000 club members signed a formal motion against him.

Bartomeu’s term of office expires in March regardless of that process, but his opponents are desperate to make him leave as soon as possible – partly in the hope of persuading Messi to stay beyond this season.

A great strength of Barca’s ownership model – the club belongs to its 140,000 members – is that ordinary fans can exert a genuine influence over the running of the club (imagine, for example, if Manchester United fans were able to force out their owners the Glazer family in the same way Bartomeu is being ejected from the Camp Nou).

But that also inevitably leads to endless rounds of political in-fighting, and those bitter squabbles could form an unpleasant backdrop to a challenging season which is threatening to unravel before it has even begun.