Brighton & Hove Albion manager Roberto De Zerbi said the lack of experience in his young squad cost them a victory at home after the Premier League club were narrowly beaten 3-2 on their European debut by Greek side AEK Athens.
Brighton, who finished sixth in the English top-flight last season and qualified for European football for the first time in the club's history, missed the experience of defender and captain Lewis Dunk through injury in the group Europa League stage fixture.
De Zerbi said he was proud of his team's performance regardless of the result.
"We were unlucky; I think the result is unfair," the Italian said. "I'm really pleased with the performance, we played very well with personality.
"Maybe we don't have the right experience, we have younger players, and we can pay (the price) for this.
"For us it was a really historic day. For our fans, I'm sorry for them. We can decide how we are going to play, but we cannot decide the result."
Brighton face a difficult task in their bid to progress past the group stage, where they will also come up against former champions Ajax Amsterdam and three-time finalists Olympique de Marseille.
Brighton will next host Bournemouth on Sunday in the Premier League, followed by a visit to Chelsea on Wednesday in the EFL Cup. They sit fifth in the league with 12 points from five matches, one point behind fourth-placed Arsenal.