"There are no negotiations with Manchester United," Barca's sporting director Ramon Planes said. "He is a player we are counting on."
After Barcelona's disastrous finish to last season, Planes, who was named in his post in August, has been conducting a squad overhaul with new manager Ronald Koeman.
The 23-year-old Demebele, who has suffered a series of hamstring injuries since arriving from Dortmund in 2017, has been linked with a move away.
"We hope to be able to take advantage of his footballing talent. I am convinced that he will have a great year with us", Planes told a press conference to introduce teenage Dutch defender Sergino Dest.
"There are four days left of mercato, it's an atypical mercato. A lot can happen," Planes added.
The transfer window closes on Tuesday (AEST), and Planes signalled that two French defenders, FIFA World Cup winner Samuel Umtiti and 20-year-old Jean-Clair Todibo, could be following Luis Suarez, Ivan Rakitic and Arturo Vidal out of the club.
"For Umtiti, we wish him all the best. He is a great player, but at the moment he is going through a bad situation, which is typical of injuries in football," Planes said.
"Jean-Clair has several offers. He wants to have time to play and grow up in another team, and that must be respected."