Bartomeu said in an interview with Barca TV that negotiations with the Dutchman are advanced but that a deal had yet to be signed.
"If there is no change then we will announce Koeman as our next coach, and he will lead this team and these players forward with a different type of project, led by a coach who all Barca fans know," Bartomeu said.
Bartomeu also addressed the future of star forward Messi, who cut a frustrated figure in the loss to the Germans.
"Messi wants to end his career at Barca, he has said that many times. I've spoken to Koeman about him and he has told me Messi will be the pillar of his project," Bartomeu said.
"Messi has a contract with Barca until 2021. I speak to him and his father regularly, and they know that we have a solid project, that a new coach is coming and that he will count on him."
Along with Messi, Bartomeu also said a number of other players are not for sale this summer.
"Messi is the best player ever and is not for sale," the president added. "But there are others who won't be sold, some of the younger players. [Marc-Andre] ter Stegen, [Clement] Lenglet, [Frenkie] de Jong, [Nelson] Semedo, [Ousmane] Dembele and [Antoine] Griezmann."
However, Bartomeu omitted a number of the club's longest-serving payers from that list -- including Gerard Pique, Jordi Alba, Sergio Busquets and Luis Suarez -- and he says it's time for some of them to move on.
"We're talking about a generation of players that have won everything," he said. "I congratulate them for that but now it's time to say an honourable farewell to some of them.
"We will speak with them first out of respect. They have been legends so they have to go in the right way, but there are conversations that need to happen."
Koeman, in collaboration with Bartomeu and new technical director Ramon Planes, who replaced the outgoing Eric Abidal on Tuesday, will be involved in the overhaul of the playing staff.
As a player, Koeman helped Barcelona clinch its first European title by scoring an extra-time winner in a final nearly two decades ago, and now the former defender is set to return to try to lift the club from its current crisis.
The 57-year-old Koeman had been in charge of the Dutch national team since 2018 and had a contract that would end after the 2022 World Cup.
Information from ESPN correspondents Sam Marsden, Moises Llorens, The Associated Press and Reuters was used in this report.