Messi followed up winning a record sixth Ballon d'Or by recording his 35th LaLiga hat-trick in Barca's 5-2 win over Real Mallorca on Saturday.
Despite spending big on Antoine Griezmann and Frenkie de Jong in the most recent transfer window, Barca are still heavily reliant on the 32-year-old, who has scored 14 goals from 15 appearances in all competitions this term.
Bartomeu has acknowledged Barca will find it extremely difficult to replace Messi whenever the forward calls time on his stay at the club, but outlined their intention to continue developing young talent from the famed La Masia academy.
"We Barca fans have the good fortune to enjoy Messi every three days," Bartomeu, who plans to offer Messi a "lifetime contract" told Italian publication La Repubblica.
"Leo is impossible to replace and when he will be here no more, we will be forced to play differently. I cannot say that we are already preparing for post-Messi, but it is true that we have already decided to bet on young players for a long time.
"For now, what we have to do is not worry, but enjoy it. I would like to make him an annuity contract as we have already done with [Andres] Iniesta. He has earned the right to decide when to say enough is enough.
"As a footballer, Messi is number one and we will fully understand it only in a few years, because Leo has changed not only the history of Barca, but that of world football."
Bartomeu also confirmed Guardiola - who left Barca in 2012 after four trophy-laden seasons in charge before going on to coach Bayern Munich and now Manchester City, will always have a home at Barca, should he wish to return.
"A possible return of Guardiola? It doesn't depend on me," he added.
"It was Pep who decided to leave, but for him the doors of Barca are always open."