Messi won his sixth Ballon d’or last week and followed that up with a brilliant hattrick as Barcelona brushed aside Real Mallorca 5-2 at the Nou Camp on Saturday.
Messi’s hattrick was his 35th in the Spanish top flight, taking him one clear of Cristiano Ronaldo in the La Liga all-time treble list.
The 32-year-old’s contract at the Spanish champions expires in the summer of 2021 with a new deal yet to be agreed.
In September, it was revealed that a clause in Messi’s contract allows him to leave for free at the end of this season.
Barcelona spent big money to recruit the likes of Antoine Griezmann and Frenkie de Jong this summer, but Messi remains the shining light of the Blaugrana.
Messi missed the start of the season through injury but has made up for lost time since his return to full fitness, scoring 12 goals in 10 La Liga appearances to top the goal king chart.
Bartomeu acknowledges finding another Messi will be difficult, and has asked for a different style of play to cope with his absence.
"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."
