Last summer, Dembele was signed as a replacement for Neymar, but the French international endured a stop-start campaign where just 12 starts and five substitute appearances were made in La Liga.
Liverpool have been linked with a summer bid for Dembele, but Bartomeu has predicted that the 21-year-old - as well as Philippe Coutinho - will have a big season for the club in their second year with the Catalan giants.
Bartomeu told The AP: "[Philippe] Coutinho, [Ousmane] Dembele are for us a big investment. They are very important players and we expect a lot from them.
"Coutinho probably in the last four months has been very active for the team. But Dembele is a very young player, a very talented player that we are, of course, expecting a lot from.
"He's been demonstrating during the season that he's a good young player for us. But, of course, next season will be different. It's not easy to play in Barcelona the first year because the model of football is quite different from the rest, but after one year I'm sure we are going to see Dembele and Coutinho at a very high level."
Dembele penned a five-year deal when signing from Borussia Dortmund last summer.