Messi has come in for flak for his form in a damaging streak of three straight defeats for the Catalan club that has seen Barca eliminated from the UEFA Champions League, lose the Copa del Rey final to arch-rivals Real Madrid and slip off the pace in the Liga title race.
The Argentina international is under contract at Barca until 2018, but has long been expected to sign an improved deal at Camp Nou.
And Bartomeu is adamant that the club will offer the 26-year-old an extended contract, defending Messi's performances this season.
"We are being unfair to Leo," Bartomeu told Esport3. "Our club members need to understand that we must take care of Messi.
"He has made such an important contribution to this club and he will continue to do so.
"That's why we have no doubt that he will lead the future of the club. He is young, he is the best player in the world, and we believe in him.
"We want to improve it (his contract) because he deserves it and to adapt it to football's current circumstance."
