Barcelona will be without both of their left wingers Marcus Rashford and Raphinha for the Copa del Rey semifinal first leg clash at Atletico Madrid on Thursday.
Raphinha is still sidelined with a thigh strain, while Rashford did not train on Wednesday.
The England international is suffering from a knee problem after receiving a knock in Barca's 3-0 win over Real Mallorca on Saturday in LaLiga.
"Marcus is not able to play, he got a hit and it's painful and we have to take care about this," Flick told reporters on Wednesday.
"I think this is not good news, but in the end I believe in my team always.
"When it's like this, the situation is not easy, we grow and this is also necessary for tomorrow."
Flick said the Copa del Rey holders had to go "step by step" with Raphinha's recovery.
"He's a player who is always giving everything, always (playing at) high intensity in the game. And when you feel something, we have to take care and we have to maybe go one step back," said Flick.
"This is at the moment the situation. And of course, I'm not happy with the situation because we need him."
Flick could use Ferran Torres or Dani Olmo out of position on the left wing to try and fill the gap at Atletico's Metropolitano stadium.
The Catalan giants also have Pedri, Gavi and Andreas Christensen out with longer-term injuries.
Barca defender Ronald Araujo, who had a month-long mental health break towards the end of 2025, admitted that he has been suffering from anxiety and depression over the last 18 months, in an interview with Spanish newspaper Mundo Deportivo this week.
Flick said the Uruguayan centre-back was "very strong" but it was also up to coaches to look after the players.
"He is very strong, so to (be) open (about) this means you have to be strong in this moment," said Flick.
"It's all about us who are responsible also, for the players. We have to take care of them. We have to take care, also maybe the opposition's coaches the same."