Just over a month ago, O'Neill watched Midtjylland gatecrash Nottingham Forest's European return with an enthralling 3-2 encounter.

The veteran boss said he took heed of that performance against his former club and was prepared for a physical meeting with "a very, very good side".

While he might have taken note of Forest's failings, his players didn't seem to.

Mikel Kruger-Johnsen scooped in a scrumptious second, but the two goals the 19-year-old sandwiched were not so unstoppable. But Celtic couldn't prevent them.

Anthony Ralston failed to prevent Kruger-Johnsen's routine clipped ball to the back post for the opener, before the teenager weaved through for the second.

Celtic as a team failed to clear their lines from a throw-in for the hosts' third.

"In terms of defending, I've just said to the players in there, some things change in the game, other things don't change," explained O'Neill.

"Once a winger takes you on at full-back, you've got to stop him from getting into the penalty area. You have to engage him before he gets there, because once he gets in there, then he's as safe as anything.

"Then in the second goal, we had a 2v2 situation, and we allowed the player just to come inside and bend it into the net.

"So from our viewpoint, not good defending, really."

The need for O'Neill to explain the concessions in such layman terms speaks volumes. Damning volumes.

While Midtjylland were majestic, Celtic didn't exactly make them work hard for their money.

"The third goal almost puts it beyond you, so was I surprised? I don't know whether I was surprised [or] disappointed, just disappointed in the concession of the goals," the Northern Irishman added.

"I think just to play football at the top level, not only do you need ability, but you need mentality, and mentality sometimes overrides ability as well.

"It sounds from here as if I'm like a teacher telling them, but I will try and teach them the game as quickly as possible.

"Things that they may already know, may need reminding, maybe they don't know, and it's my job to try and improve the football club."

The overriding questions remains, though. How long will O'Neill have to improve Celtic?

His return, alongside assistant Shaun Maloney, has brought back a feelgood factor but that was only going to last so long.

The laughs had over O'Neill's matchday fit have faded, while Celtic's deep-rooted problems have returned in stark fashion.

Captain Callum McGregor was at the heart of the happiness on Sunday, scoring in the extra-time win, but he was quick to assure no-one had got carried away.

"Nothing's been solved after a really good game at the weekend," the midfielder said after defeat in Denmark. "We know that we don't get too up or too down.

"We come away here against a really good side, a good club, who do a lot of good things and they know what they are.

"There's a lot of growth still left in our team as well. We know where we are and we know where we want to get to."

It appears Celtic are far from the latter, and it's lined up to be an almighty task to get them there, for whoever is charged with taking them there.

On a sobering night, it's not the interim manager who will take the heat. It's not even the players being taught by him.

It's the board, who have managed to quieten the clamour aimed in their direction for a few days with the reinstatement of O'Neill, who will be feeling the pressure once more.