When Lionel Messi won an unprecedented sixth Ballon d'Or in December last year, the speculation had already begun over his career at Barcelona after he sought to trigger an exit clause and leave as a free agent.

An April consolation prize of a Spanish Cup success -- his 35th club trophy -- was scant reward.

The 34-year-old Argentine footballing genius has countless times over 17 years with the Catalan club been able to propel the side to glory.

But the club has been imploding off the pitch amid a soaring debt mountain.

Now there are fears for the future on the pitch after the club said Thursday that agreement on a new deal could not be reached "due to economic and structural obstacles" under Spanish League rules, scotching plans for him to remain with his only club for another five years, until 2026, when he would be 39.

The 34-year-old signed his first contract with Barcelona in 2000 on a napkin when he was just 13 but his contract expired on June 30, meaning he could look once again further afield.

In truth, he has been eyeing different horizons for at least two years, however.

Barcelona, despite lifting the La Liga title, collapsed at the end of the 2018-19 season, losing a 3-0 semi-final lead, including two Messi goals, to Liverpool in the Champions League.

They then lost the domestic cup final to Valencia.

Bayern embarrassment

If that was bad, worse was to follow in a barren 2020, capped by a shocking Champions League quarter-final humiliation, 8-2 to Bayern Munch in Lisbon.

That abject showing meant a first trophyless season since 2007 and also marked the first time Barca had conceded eight goals in a game since losing to Sevilla 8-0 in the 1946 Spanish Cup.

A teenage Messi celebrates after scoring against Real Athletic in 2006. Photo: AFP

More significantly, the loss convinced the twinkle-toed talisman his final footballing years lay elsewhere.

The fallout was immediate. Coach Quique Setien was sacked after barely six months in charge and former Camp Nou fan favourite Ronald Koeman came in -- only for Messi to inform the club he wanted "unilaterally" to terminate his contract by triggering a release clause.

Now once again, he is headed for the exit door even if few clubs are likely to have the financial firepower to pay his wages although he is at least available on a free transfer.

The suitors with the deepest pockets are Manchester City, led by Messi's former Barcelona mentor Pep Guardiola -- or Paris Saint Germain, where he would link up once again with former teammate Neymar.

City were agonisingly close to European glory last season -- as were PSG the year before -- and the addition of Messi could give Guardiola or Mauricio Pochettino the X-factor as both pursue the one trophy missing from the cabinet.

The growing turmoil enveloping Barcelona has been reflected in Messi's diminishing statistics. 

Though he managed 31 goals in all games during the virus-interrupted campaign just ended, it was his lowest return since 2007-08.

Although he hit 30 last year to top the charts to take his club record to 474 it was well off his peak year of 50 league strikes in 2012.

That season he set a European all-time record season tally of 73, breaking Gerd Mueller's 67 goals scored in the 1972–73 German season.

Irreplaceable

Barcelona have insisted in the past they had plans for coping with Messi's retirement, whenever it came, but the truth is their number 10 is irreplaceable -- on the field as well as off.

According to Catalan financial firm Diagonal Inversiones Messi is worth more than his salary -- by a distance.

Messi holds his head in his hands during a Champions League game against PSG. Barcelona have disappointed in the Champions League in recent years. Photo: AFP

The father of three -- married to childhood sweetheart Antonella -- may have earned some €385 million in the past four years at Barca.

But a study by the firm indicates he generated income of €619 million in that time for a €235.6 million profit. He is also reportedly responsible for 30 percent of club income and eight in ten shirt sales.

Take that out of the equation and Barca will be losing more than just a man who led them to 10 La Liga crowns, four UEFA Champions Leagues, three Club World Cups and six Spanish Cups and who has scored the most goals and most hat-tricks in La Liga while netting more than 700 senior goals for club and country.

For Messi, one more golden hurrah is the hope.

For Barca, who style themselves "more than a club" if this is indeed adios to a superstar who is 'more than a player' his departure will leave a massive hole to fill on the pitch as their number ten seeks pastures new -- and a gargantuan one for their accountants to plug. Somehow.