Pierre Sage said Crystal Palace were "really far off" his expectations after suffering a 3-2 Europa League defeat away to his former side Lyon.
Corentin Tolisso opened the scoring for the hosts and Ernest Nuamah doubled Lyon's advantage late in a first half void of any shots on target for the visitors.
Yéremy Pino cut the deficit in half five minutes after the break before Pavel Sulc restored the two-goal lead, and substitute Daichi Kamada's late goal proved a mere consolation.
"At the end, we were frustrated by the result, but in the first half we have to be frustrated in the way we play," Sage said to PalaceTV.
Pierre Sage was left frustrated by his team's poor performance in the first half. Photo by Daniel Weir/Sports Press Photo/Getty Images "I think we didn't play this first half. We are really far off our standards in the mental side of the play, in the tactical side of the play, because the players are too high between our lines and we really struggled a lot with that.
"When you give back the ball to your opponent, you suffer a lot and we were better in the second half. You cannot win this kind of game by playing only one half."
Sage's side got the better of his predecessor, Oliver Glasner, after they bounced back to beat Nottingham Forest 3-1 at Selhurst Park on Sunday.
After that contest, the Frenchman called for more consistency from his side, but on Thursday night was left rueing missed opportunities and goals conceded too easily by last season's Conference League winners.
Sage was sacked by Lyon in January 2025 after a 14-month tenure with the Ligue 1 outfit. Like Glasner's, his own reunion ended in disappointment.
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He told UEFA: "It's a very disappointing return because we lost. I can't take satisfaction from coming back home or seeing familiar faces when the result isn't there.
"The scoreline may have been close, but there was a big difference in the way the two teams played.
"We improved considerably after the break and gave ourselves a chance, but conceding that third goal made things difficult.
"I think our second-half efforts deserved an equaliser, although winning would have been a generous reward given that we only really played [to our level] for one half."