Celta made a promising start at Balaídos on Thursday, with Pablo Durán giving the hosts an early lead. However, Besfort Zeneli scored twice to turn the match around, leaving Giráldez's side with just three points after four league-phase fixtures.
The Celta Vigo manager felt his team had done enough to take something from the contest, particularly after producing a dominant second-half performance.
"We've played four good matches in Europe, but the reality is that we have three points. The level of performance in these European games is what we have to maintain," Giráldez said.
"The first half was competitive and in the second half we were far superior. I think it was a fantastic match. I think the fair result would have been a victory for us, or at least a draw, but we're leaving with nothing."
Giráldez was particularly frustrated that Celta could not make their opportunities count. After taking the lead through Durán, the hosts were unable to establish a greater advantage before Zeneli's first equaliser.
The Belgian side then scored again shortly after the interval, leaving Celta chasing the game. The home team had opportunities to rescue a point, but could not find another goal.
"Football is about taking your chances and they did that. We had the opportunity to finish them off when it was 2-0 and then we had the chance to equalise in the final minutes, but we came away with nothing," the manager said.
Despite the disappointment, Giráldez refused to lose confidence in his team's prospects. Celta have four matches remaining in the league phase and he believes their performances justify optimism.
"I'm fully convinced that we're going to be in the next round. We have four matches left and it's not normal that, playing like this, we haven't managed to get points."
He added: "We can beat all four of the opponents we have left, and I hope everyone sees it in the same way and feels as proud of the team as I do."
Celta's immediate challenge is to turn encouraging performances into results. Giráldez believes the team's level has been good enough to compete in Europe, but the defeat against Union SG leaves little room for further missed opportunities if they are to progress.