The 18-year-old has been struggling with a persistent groin injury since the start of the season, with Barcelona pulling him out of the Spain squad for the upcoming international break.
Yamal picked the injury up last time he was with the national team but made his return against Real Sociedad and even played the full 90 minutes in the 2-1 Champions League defeat to PSG.
Speaking ahead of their La Liga clash with Sevilla on Saturday (October 5), Flick admitted the prognosis was ‘not good’ and cast doubt on whether he would be fit for the upcoming El Clasico.
Flick said: "I spoke with him today, it's better but it's not good. This is the situation with Lamine. With this injury, it's not so easy to know (when he will return).
“It's not easy to say 'OK he can play in two, three or four weeks...' or if he's able to play the Clásico. This is not possible. We have to wait.
“We must manage his load. He trains with the recovery team and (spend) the next weeks with them. We go step by step. We have to see how the evolution is."