Flick brought through the 18 year-old midfielder over the past 12 months at Barca, but the club lost him to PSG.
Dro threatened to walk out on Barca for his €6m buyout clause before PSG agreed to pay €8.5m to maintain their relationship with the Catalans.
Ahead of their clash with Eder Sarabia's Elche this weekend, Flick said today: "I have changed my mind. I don't want to say anything about it.
"I love Dro, but I'm obviously disappointed. He knows it too, but I love him. He would have had a great future here.
"He made a different decision, which I have to respect, this is football..."
Flick, meanwhile, says he's happy with Marc Bernal's return after last year's knee injury.
He stated: "I'm sure that Marc Bernal he is a vital player for the future of this club. He is a player with a lot of quality, but he is young and is coming off a serious injury.
"What matters is that you have to go step by step. Everyone wants to play more minutes, but you have to be patient and improve. This is the right philosophy."