Dembele has reportedly emerged as an alternative to Borussia Dortmund star Jadon Sancho for Premier League giants Man Utd, who are determined to bolster their squad before Monday's deadline.
Barca head coach Koeman was asked about Dembele post-match and he said: "What is certain in the world? I don't know.
"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."
Barca's 100 per cent start to the season was ended by Sevilla following a draw at Camp Nou, where Luuk de Jong and Philippe Coutinho traded goals.
De Jong opened the scoring for Sevilla in the eighth minute, but Barca star Coutinho restored parity just two minutes later.
Barca and Sevilla both have seven points from their opening three matches, three points adrift of leaders and champions Real Madrid.
"It was a difficult game," Koeman said. "They pressed us well. Our problem was that our play with the ball wasn't how we would like.
"They play good football, they press well and physically they're stronger than us. They're a top team.
"I think we played well, but we lost the ball a lot in the second half, so it's difficult to get into a rhythm in the game.
"I'm happy with the team's performance. The team was tired, but we fought until the end. But [we played against] a great opponent."
Antoine Griezmann produced another underwhelming display and he was the first player withdrawn by Koeman just past the hour-mark, like he was against Celta.
"The change the other day [against Celta Vigo] was tactical," Koeman added. "With Ansu [Fati] we had more penetration and pace - something we needed in Vigo with 10 men.
"Today, [Griezmann] had two opportunities [to score] and, for a player of his quality, he should've scored either of those two chances."