The Spanish champions agreed a £65million deal to sign 21-year-old Ajax midfielder Frenkie De Jong last month, as well as teenage defender Jean-Clair Todibo.
Brazilian duo Arthur (22) and Malcom (21) were also signed in the summer.
Bartomeu says their additions is to prepare the squad for the day that Messi decides to leave.
Speaking to journalist Guillem Balague on BBC Radio Five Live’s Football Daily Podcast, he said: “I know one day Lionel will say he’s retiring.
“We have to prepare the club for the future.
“We are bringing great young players to the team because we need to continue this era of success. That’s our responsibility.
“My mandate will be finished in two seasons. So I need to leave the club in a great position and tell the new president ‘that’s our legacy’.”
