Barcelona currently sit top of the La Liga table, four points ahead of arch-rivals Real Madrid, with 70 points from their 31 games.
Flick’s side have nine games across all competitions in April alone, including the first leg of their Champions League semi-final against Inter Milan and the Copa de Rey final vs Real.
Speaking to the press ahead of the game with Celta Vigo, the German manager unleashed a furious rant at the scheduling.
“Do you know how many matches we have played in recent weeks? And do you we play late against Valladolid? We arrive very late, maybe the players are in their beds at four or five in the morning.
“I don’t want to make excuses. But it happens here, and I’ve never had this before. Every league protects their clubs in the Champions League, and especially in the semi-final, it’s unbelievable.
He continued: “And we have no time to rest, and I want to discuss with the guys who are responsible. Because they have no idea how it is. No idea how it is.
“It’s just one thing. I don’t want to come here and complain or make excuses, and OK I’m happy it’s not Sunday at 14:00, as it was before, now we are playing on Saturday at 21:00 in the evening.
“Why can we not play at 18:00 in the evening? Give me one reason. I want to see this guy that decides it, who is responsible for it. For me, it’s a joke. I think also for bad for Spanish football.
“If we are successful, the other teams in the Europa League, it’s good for Spanish football. Then we have one more team that can play in the Champions League. I am speechless. I have no words for that. It’s unbelievable the situation.
“Every Federation, the Bundesliga, Premier League, takes care of their clubs, but we don’t do it here. It’s just, ‘Ah, let them play, they can come back at five or six o’clock (in the morning).’ We have just reached bed for four hours. It’s unbelievable.”