Marca says on Monday, the 23-year-old turned up at Barcelona's training ground at 10:15 CEST when the limit is 10:00, as Ronald Koeman wants them to be present an hour before the start of the session.
Dembele has had problems with discipline before and has been late to sessions on more than one occasion this campaign and in previous ones.
In fact, there was one occasion during the 2018/19 season when he was asleep and didn't turn up to training.
Koeman is a strict coach who attaches great importance to discipline and group arrangements, as he believes this helps the team in the long run.
For victory over Villarreal, Dembele came on for Ansu Fati in the 70th minute.