The nature of Sunday's League Cup defeat by Rangers brought significant scrutiny and criticism. A reaction was demanded and O'Neill made four changes.
Initially, things looked good. James Forrest caused problems down the left and Ferencvaros looked on the back foot. But goals change games and this was a prime example with Celtic's fragility brutally exposed.
Against the run of play, Yusuf cut in from the left, whipped toward the back post, and Liam Scales could only help the ball into the far corner. It was a cheap concession.
Baur, celebrating a first call-up for Germany, finished off a fine move to level on the stroke of half-time but, within seconds, Celtic had allowed Zachariassen to flick in amongst static defenders.
O'Neill changed shape at the break. Kasper Hogh joined Duran up front. But almost immediately, Celtic were further behind.
Scales' loose pass was punished by Arzani, who produced an outstanding finish. A dream moment for him after an injury-ravaged Celtic spell.
There was no route back into the game, Benjamin Nygren coming closest, and Celtic's bad week got worse, with the prospect of Rangers at home on Sunday looming large.