Brendan Rodgers said his Celtic side played like "under-12s" for a spell in the first half of their 5-0 Champions League hammering by Paris St-Germain.
The Scottish champions were 3-0 down at the break in their Group B opener.
Rodgers said he did not want to be too critical of his players but felt they were off the pace in the first half.
"You have to look after the ball and make it work for you and, first half, we didn't do that - we played like under-12s at times," Rodgers said.
"It is confidence, it's belief. We are obviously missing some decisive players that can help us at this level, but it is always going to be very, very tough. That is the reality."
Rodgers said he did not want to be "too tough" on his players as they were playing "the potential champions".
He added: "With all due respect, where these guys are at it's not just one or two levels, it's three or four levels beyond us.
"But what we do have to do is always be competitive and we weren't for that first half.
"The second half, we were much better and we showed the supporters that - and they acknowledged that."
The visitors went in front thanks to a goal from Neymar, the world's most expensive player, before Kylian Mbappe made it 2-0.
Edinson Cavani scored from the penalty spot after Jozo Simunovic hauled him down before Mikael Lustig turned the ball into his own net and Cavani added a fifth.
It was Celtic's heaviest home defeat in European competition, and Rodgers was left "very disappointed" with his side's first-half showing.
"Defensively, we were just too far off them, not tight enough and not close enough," he said.
"I was disappointed in how we retained the ball - from our midfield to our front players, we didn't keep the ball long enough.
"If you do that against what is arguably the best team in the world then they will punish you.
"I was much happier with that first half-hour of the second half. We were much more aggressive in our game and that allowed us to create more chances.
"But you also have to give credit to them - this is a team that is really special. They are going to be close to winning it for sure."
With PSG having spent £200m on Neymar, and having agreed to pay £165m for on-loan Monaco striker Mbappe, Rodgers know his side face a huge challenge in Europe.
However, he remains hopeful his charges can still enjoy a solid campaign.
"The thing I said to the players is, 'you have to have the belief' and, maybe a bit similar to last year, it might take us a game or two to get into the competition," he said.
"Then we recognise the level we are at and then you perform better."
PSG coach Unai Emery was delighted with the way his players coped with the atmosphere at Celtic Park.
"The most difficult thing here is to cope with the rhythm Celtic play at and the intensity they play with and, of course, the support they have with them," he said.
"I think my team showed a lot of character. It was really a team effort and it showed a really controlled approach to the game, which I'm very pleased with."