Fati has been in excellent form so far this season but was dropped to the bench for Barcelona's 2-0 win away at Juventus in midweek, and Koeman has stated that the 17-year-old needs to work on one part of his game in particular.
"I would advise him to continue working and improving," Koeman noted.
"Yesterday I talked to him about improving his concentration. His ball losses are due to a lack of concentration, not quality.
"He has talent, he is a very good player. He has potential and we must help him to improve. He must be concentrated to improve."