Barca won through goals from Gerard Martin, Marc Casado, Ronald Araujo and Robert Lewandowski. La Real had Aritz Elustondo sent off early in the first-half.
Flick was happy with the win and also seeing Barca back on top of the LaLiga table.
Victory:
"I'm very happy. The red card changed everything but, in the end, we forced them. They are a very good team, but we deserved it.
"We talked about keeping a clean sheet. We are capable of scoring. Maybe in the first half we could have scored one or two more."
Casado goal:
"Very happy for him and for Gerard Martín."
Lewandowski:
"It's always important that he scores goals and today he had some luck, but we're happy.
"We always want to play and attack, pass the ball to attack and everyone is hungry. That's a good mentality. Today we had an ideal situation. What we want is to get to the back line. But we are happy with the goals we scored."
Gavi:
"Yesterday he had a fever and we have to take care of him, that's it. We talked and we came to the conclusion of not taking him out and playing with someone else. It wasn't the option. We thought it was best for someone else to play. Araujo had to come off. Sometimes you have an idea, but sometimes you have to adapt.
"Things can change very quickly. It's very important for us to stay focused. There's still a long way to go in La Liga, the Cup and the Champions League... but we always fight for all the titles... we're in the situation we're in and we want to fight."
Benfica:
"I think there are many things that have helped us today. The red helped us, but we also played well. Although it is true that the way today's game went allowed us to rest some players in the second half. But they have had a week's rest."
Unbeaten streak:
"I'm not surprised by the performance of this team. After the defeat against Atlético in December I said we were playing well. We played well, we are focused, we have possession, the defence has improved... we learn from each game."
Gerard Martin:
"He played extraordinarily well today. It's nice for them, they deserve it. He's improved. They have confidence. Balde had a lot of minutes."
Lamine Yamal:
"It's good that he always makes the final passes. He prepares himself, creates opportunities... He does it very well. The day will come when he scores. He always tries to create things, generate opportunities... the opponent does it well too. I love Real Sociedad. It doesn't matter if Lamine scores or not. We are a team and when someone scores we are all very happy."