Ante Budimir (2), Bryan Zaragoza and Abel Bretones struck for Osasuna, with Barca's goals scored by Pau Victor and Lamine Yamal.
Afterwards, Flick refused to seek excuses for his players for the setback.
First defeat:
"You have to accept these defeats. We haven't played very well. I think it's my responsibility to protect the players, because they've been playing a lot of minutes. But I didn't expect we'd play like this. I've told the team that we have to keep going. We play on Tuesday, on Sunday... and we're on the right track."
Performance:
"We made a lot of mistakes in the first half, but in the end we made changes. If anyone is responsible, it's me, but I have to look after the players. I think we're on the right track. We have two days to recover. I hope we see a different team tomorrow."
Thrashing:
"We made a lot of mistakes and they did very well. Although I think there was a foul at the start of the second goal, I'm not entirely sure because I haven't seen it yet, but that's what I heard. For them to score four goals against us is too much."
Changes:
"I think that in the second half, we tried to win. We had opportunities. But today Osasuna did very well, in the transitions. They gave us problems. We have to analyse it and do better."
Rotations:
"If you look at our team, it's not easy. I think that after the break, we will have more players fit. There are a lot of injured players, but it's not an excuse. In the second half, we played better. We combined and we did better. If we had done it in 90 minutes we would have finished better."
Robert Lewandowski's substitution:
"Because he also needs rest. That's how it is. Everyone needs rest. It's tough, but every three days we have a match and we do it for his own good."
Concern?
"I'm positive. I think we have to get our first win in the Champions League."
Koundé fouled and possible penalty on Lamine Yamal:
"I'm sitting outside and I don't have a good view. It could have been a foul, but it's the referee who decides, and there's also the VAR. Things are like that and I don't give it any more importance."