The relationship between Messi and Bartomeu reportedly became toxic during the latter's final months in charge at Camp Nou, and Joan Laporta's election win in March 2021 appeared to point to a brighter future for the team.
However, in August there was the bombshell announcement that Messi would be released as the club could not afford to retain him, with its finances having been hit badly during the pandemic.
An agreed contract had to be discarded and Messi joined Paris Saint-Germain, which has no such money worries under its Qatari owner.
There ended Messi's 20-year career at Barcelona, during which he became the pivotal player. Messi departed as the club's record scorer, with the most first-team appearances of any player in Barcelona's history, and against his own wishes, a sobbing superstar waving an unexpected farewell.
"I have always thought that it is essential that he should be with us, not only because he is the best in the world but also because of his economic and institutional contribution," Bartomeu said, in an interview with Mundo Deportivo.
"It is a mistake to let Messi go. He represents much more than a footballer that you fall in love with."
Messi attempted to leave Barcelona after the 2019-2020 season, believing an agreement that he should be allowed to depart if he told the club of his intention by 31 May should have been allowed to carry over until the end of August, given the delay in the campaign caused by the COVID-19 crisis.
That was rejected by Barcelona, with Bartomeu fighting to keep Messi with the Catalan club.
"He wanted to leave the club, we talked about it and I said no," Bartomeu said. "I have always thought that Messi is very important for our club, also Barca is for him and it would be a serious problem if he left, as I think it has been now.
"I told him that if he wanted to go like Xavi and [Andres] Iniesta, to Qatar, China or the United States, something we understood, we can talk about it and we will do a tribute and a farewell.
"But Messi didn't have a team yet and he wanted to be free. We told him: 'We want Barça to be your last club in Europe. If you want to go to another continent later, no problem. But we want you to continue,' and that was a bit of the story of the summer of 2020. Us telling him that we wanted him to continue and he, that he wanted to leave. But without knowing where. I always asked him where he wanted to go."
Bartomeu frowned on suggestions there had been no strategy for growing the team, which was said to have been a major complaint of the Argentina forward.
He also dismissed any suggestion that players had an overbearing influence, having admitted Messi and Luis Suarez promoted the idea of bringing in Philippe Coutinho from Liverpool, before he arrived for an eye-watering fee in January 2018.
Barcelona agreed a deal said to be worth up to £142 million ($264.6 million) for the Brazil midfielder, whose signing has not lived up to expectations.
Asked about player power, Bartomeu told the Spanish newspaper: "In the dressing room they have a power and you have to listen to them. It does not only happen at Barca.
"You have to take them into account, they know the day to day and during this term we signed many players. Sometimes we have taken their considerations into account and sometimes not."