Dembele continues to be linked with Manchester United.
"What is certain in the world? I don't know," Koeman said when asked about the Frenchman's future.
"At the moment, he's a Barcelona player.
"There's still one day left for incomings and outgoings. We'll see.
"I opted for [Francisco] Trincao, Pedri and [Miralem] Pjanic [off the bench].
"We needed more control and that's why I made these changes."