The clouds around Celtic darkened further as they followed Old Firm defeat with a loss in their Europa League opener against Ferencvaros.

For the second game running, Martin O'Neill's side conceded three times and looked fragile throughout - just as they did in their last European game against LASK in the Champions League qualifiers.

Despite a bright start, the Scottish champions were ruthlessly punished for slack defending, as strikes from Bamidele Yusuf, Kristoffer Zachariassen and former Celtic winger Daniel Arzani gave their Hungarian visitors victory.

Mika Baur had briefly levelled just before the interval but Celtic's deficiencies were laid bare, conceding almost instantly to hand back the advantage.

Changes in personnel and shape made little impact on the outcome and leaves Celtic's Europa League campaign already looking badly damaged. Sterner tests lie ahead, with a trip to Portugal to face Benfica next.

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.

The good part for Celtic was the start. Full of attacking intent with Forrest central to it down the left. Tempo, intensity, control and no little skill.

The bad came defensively, again. One of a number of concerns was how many self-inflicted errors caused havoc.

There is clear evidence to support the idea that Celtic are incredibly fragile at the back. The answers to solve that don't seem obvious.

At this level, they were ruthlessly exposed. Imagine the scenes against potential Champions League opponents had they got there? O'Neill's side didn't ​get there for similar reasons.

There was so much focus on what Celtic's line-up would be. The four changes didn't really work. A change of shape at the break was undermined by Arzani's goal.

Celtic pressed on but it was too little too late. New signings, in the main, just haven't stepped up in these big games. That's a concern.

O'Neill's magic touch seems to have deserted him in these big moments and the questions will now get sharper.

Just what is Celtic's preferred starting XI? Or the best shape and system to maximise the talent they have, while providing stability at the back?

There are any number of solutions required ahead of Rangers' visit on Sunday or a similar outcome to last week may follow.

Celtic manager Martin O'Neill: "We're naturally disappointed. We started off very, very brightly. We had a number of balls that were flashed across the goal and no-one there to put the ball in the net for us.

"Their first proper attempt yielded a goal for them. We come back into the game, got the equaliser just before half-time and then conceded a really poor goal.

"We're hit on the break for the third goal. While we had a number of balls flashed across the box even in the last 20 minutes, we didn't get there to the front post a number of times that could've kept it alive and we're beaten in the game."

Celtic captain Callum McGregor: "It's a sore one but we have to stick together now. It's a little bit broken, a little bit nervy, but we have to fight our way through this. The only way through it is hard work.

"It's a tough period. You can see how quickly it comes down on top of you. It's a relatively new group, so we have to learn. You don't get much leeway.

"The ultimate test at the weekend is that we have to win the game. It's about staying together, putting one foot in front of another and we just need another bit of luck."