Ange Postecoglou was left to rue a series of missed chances after Celtic slipped to a 4-3 Europa League defeat to Real Betis on Thursday as he became the first Australian to coach in the group stages of a major European club competition.
Celtic took a two-goal lead in Spain but Betis hit back with four goals in 21 minutes before Anthony Ralston's 87th minute effort gave the Glasgow side hope late on.
"It's mixed emotions," Postecoglou said. "I'm really proud of the players. I think against a quality opponent, we took the game to them and had our moments where we could have got away from them.
"Unfortunately we didn't do that and they've got some quality up front that punished us.
"The players hung in there until the end and I thought it was a good game."
Postecoglou was a surprise replacement for Neil Lennon ahead of the current campaign after Celtic failed to retain the Scottish Premiership title last season, and the Asian Cup-winning coach said his team were a work-in-progress.
"The football we played was exactly the sort of team we want to be, but we've still to improve," he told Celtic TV.
"When you come away from home, score three goals and create many more chances apart from that, I think moving forward there's some good signs there.
"I don't think there was much between the teams," he added.
"We probably had an equal amount of chances but they took one more of theirs than we did.
"As I said before the game, we're trying to build resilience with this group. We don't want to make excuses; we want to be a certain kind of football team, irrespective of the circumstances and that's what the players showed today."
(Reporting by Michael Church, Editing by Peter Rutherford)