The defeat capped a difficult season for Barcelona – the first without a title since 2007-08 – and sparked one of their worst crises.
Messi, 33, won a record six Ballon d’Or during his time in Barcelona as the best player in the world and helped the club win 10 Spanish league titles and four Champions League crowns .
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However, his attempted exit from the club could now lead to a legal battle between the two sides over a clause in his contract.
Barcelona have said the document Messi sent referred to a clause allowing him to leave for free at the end of the season. However, the club said the deadline to trigger the clause expires in June and that they will seek legal advice.
The Spanish season would normally have ended in May but has been postponed due to the coronavirus pandemic.
If Messi can leave for free, there will be no shortage of suitors. However, only a handful of clubs could realistically afford to pay their wages even without transfer fees – with mega-rich Paris Saint-Germain and Manchester City widely seen as perhaps the most likely destinations.
Messi had been outspoken about his dissatisfaction with some of Barcelona’s decisions this season. He had been silent since the loss to Bayern, raising doubts about his future.
Club president Josep Bartomeu had said he was confident Messi would end his career at Barcelona and that he was a crucial part of incoming coach Ronald Koeman’s plans for the team.
Meeting with the coach last week
Koeman met Messi last week and the player reportedly told the Dutchman he saw himself more outside than at the club at the time.
The coach has reportedly already told other key elements of the squad that they were not in his plans, including Luis Suarez, Arturo Vidal, Samuel Umtiti and Ivan Rakitic.
Following the loss to Bayern, Barcelona have announced that they will make “deep” changes to the first team and a “wide” restructuring of the club. The changes started with the sacking of coach Quique Setien days after the Champions League defeat in Portugal, and new presidential elections were announced for March.
Messi arrived in Barcelona as a teenager in 2001, joining the club’s famous La Masia youth academy. He made his first-team debut in 2003 at the age of 16, and since then he has led Barcelona to 34 titles. He holds most of the club’s individual records, scoring 634 goals and providing 276 assists in 731 appearances for the club.
Former Barcelona defender Carles Puyol, who served as club captain for much of Messi’s career, has tweeted his support for the Argentine.
“Respect and admiration, Leo,” Puyol wrote. “You have my full support, my friend. “