Celtic manager Ange Postecoglou could not disguise his concern after his side warmed up for their European opener with a home defeat by Preston.

Postecoglou admitted his side struggled throughout as Ben Whiteman's penalty earned the Sky Bet Championship outfit a 1-0 victory on Saturday evening.

The Australian was fully aware of the challenge ahead of him when he took over a squad in the midst of a major transition but the slow process of signing players in a pandemic has not made his job any easier.

Postecoglou fielded three summer signings against Preston – former Sheffield Wednesday pair Liam Shaw and Osaze Urhoghide both started, while Israeli winger Liel Abada came on for the last half hour.

But those three signings have an average age of 20 and Celtic are crying out for more experience, especially in defence with Christopher Jullien still working his way back from a knee injury and Kristoffer Ajer seemingly on his way out.

A back four of Tony Ralston, Greg Taylor, Stephen Welsh and Urhoghide did not inspire confidence and Preston looked the likelier team to make the breakthrough.

Postecoglou was honest in his assessment.

"It wasn't a great game for us," he said. "The players worked hard, as they have been in training.

"We have deliberately put a lot of work into them. I guess there is an element of fatigue there and they are going into games not feeling 100 per cent.

"But that's part of the process. But it wasn't great – we didn't get a grip on the game and struggled through."

David Turnbull and Callum McGregor impressed in midfield but Celtic had two teenagers out wide in Owen Moffat and Adam Montgomery and a total of eight players under 20 in their squad.

With Leigh Griffiths absent amid a club probe into his social media activity and new signing Kyogo Furuhashi potentially two weeks away from arriving, Celtic are also short up front.

Odsonne Edouard played half an hour after recovering from a thigh strain but Albian Ajeti was largely anonymous for the first hour.

When asked if he was confident ahead of Tuesday's opening Champions League qualifier against Midtjylland, Postecoglou said: "We've been working hard to get the team ready.

"We will use the next two days to finalise our plans and hopefully we can put a good performance in on Tuesday."

There were positive signs in the final 20 minutes, with Abada forcing a save and Tom Rogic getting on the ball after brushing off a fitness issue to get off the bench.

But Postecoglou is not expecting too much, too quickly from the Israeli arrival.

"He literally just met everyone on Friday," the Australian said. "I just put him out there to get a feel for it and get to know the players. It's about letting him get to know us and vice versa.

"It was good for Tommy. He has had an interrupted pre-season, hopefully he gets some game time in the next couple of weeks."

Further encouragement could come in the shape of players who missed the game. Ismaila Soro, Nir Bitton and Ryan Christie will all train ahead of the visit of the Danes with a view to being involved, although James Forrest is set to miss out after a period of self-isolation.

"There's a couple who are close and we didn't want to risk them," Postecoglou said.

"Because James missed so much, he was out for 10 days, we have just got to be a little bit careful with him.

"He only really got a light run out on Friday, and Tuesday could be a little bit early for him. But certainly after that he will hopefully get a full week's training."