Flick admits title celebrations over the previous days had taken their toll as Barca were defeated 3-2 at home.
He said afterwards: "This belongs to the players. They have to celebrate. It's a day to celebrate.
"We played on Thursday, we celebrated, and we celebrated on Friday too... I'm happy with how we played. We had chances to win, but the most important thing today is that they feel proud of what they've done this magnificent season."
"You feel the feeling of the entire season, the connection they've had. The party was incredible."
Barcelona has felt like home
"I'm very happy. It's a happy day after winning three titles. When the season started, the feeling when we arrived was that it was like we were coming home. This whole season, everyone has connected really well. It's fantastic to feel that. We're a family, I really feel it."
Flick was also keen to highlight the influence of Bojan Krkic amid the celebrations. The former striker has acted as loan manager and also as a coach and liaison with Barca Atletic.
The German added: "He's the connection between us and Barça B, if we have to talk to other players... he's the reference, the person. He knows everything about this club. It's fantastic that he can give me this information."