Celtic chief executive Peter Lawwell accepts that the club's transfer policy has faltered in the last year or so.
He said: "We've got more players coming in right than wrong, but recently have we replaced what's left, in the striker department, with as good quality?
"That would be a question. But that's not down to lack of money, it was just judgement calls."
But striker Gary Hooper and key players Victor Wanyama and Kelvin Wilson left last summer and Celtic recently sold keeper Fraser Forster to Southampton in a £10m deal, while Georgios Samaras and Tony Watt also left before the current season got under way.
in Champions League qualifying in midweek to consign them to the Europa League where they will play Salzburg, Dinamo Zagreb and Romanian side Astra in Group D.
Some fans gathered outside Celtic Park after the defeat to protest at what they perceive to be a lack of investment in the squad.
Lawwell told BBC Scotland: "It was a very painful night but we have to move on. We, as a club, need to keep the head.
"There has been a lot of criticism. Most of it is not fair. I think there's maybe a lack of understanding at the moment about where Scottish football is and Celtic as a club.
"Our commitment as a board has been that every penny that comes in will be reinvested back into the club. That is a fact.
"In the last four years before Ronny [Deila] came in we spent over £30m. In Gordon's [Strachan] time it was £38m, in Martin's [O'Neill] time it was over £40m.
"We are not frightened to invest. To think that buying players and keeping spending cures all the ills, you need to be more intelligent than that."
Deila, still finding his feet at Celtic, has brought in striker Jo Inge Berget, midfielder Aleksander Tonev, defender Jason Denayer and forward Wakaso Mubarak on loan, plus keeper Craig Gordon, who was a free agent.
And a £2.3m fee has been agreed to sign Serbia striker Stefan Scepovic from Sporting Gijon.
"We've brought in five players this window and we're hoping to bring a sixth in over the weekend," said the chief executive.
"We could have bought them but we think it is smarter to play them for a year and see if they fit in.
"If they are right we have an option to buy them. I think that is smarter."
Despite the disappointment of their Champions League exit, Lawwell is backing the Norwegian Deila to succeed at the club.
He said: "Ronny has come in to a baptism of fire. He has new ideas and new philosophies. He is very intelligent.
"Nothing prepares you for the enormity of managing Celtic. He has come in at the transfer window and all the speculation of players coming and going and he had to qualify for the Champions League. That was a huge task.
"Chapter one of Ronny has finished with a sad ending. He will get every support to build his own team and be successful."
Lawwell acknowledges that Celtic need competitive, meaningful domestic matches. He stands by his position that Celtic could survive without Rangers being in the top league but admits that to replace the loss of their rivals they have had to be smarter in the transfer market and enjoy a European run.
"Rangers going has taken millions out the game," he said.
"We have probably lost more than anyone. We have bridged that gap by selling players and making profits and we've kept Scottish football at a level more than any other club and I think we don't get enough credit for that.
"In order to maintain the revenue stream we had three or four years ago we've had to sell players to make that gap. It's not only about buying in players, it's paying the bills.
"When you can't afford to buy the best, as maybe we could have done in 2001-03, it is now impossible to buy the best; you have to create the best.
"The main challenge is to convince the right players to come here to play for Celtic, develop them, put them in the team and get in the Champions League.
"One year we'll make money if we're in the Champions League; this year we won't make money."
Supporters fret that the defeat by Maribor could lead to the sale of Dutch defender Virgil van Dijk
"We don't need to sell him and we don't want to sell him and the player's been told that," said Lawwell.
"He is a young man with a lot of talent. We don't think it's the right time."