Celtic's failure to back Ronny Deila in the transfer market cost them any chance of qualifying for the Champions League, according to Andy Walker.
The Scottish champions' dreams of reaching the group stages for a third successive year were left in tatters after a humiliating 6-1 agggregate defeat by Legia Warsaw.
Last week's 4-1 loss in the Polish capital left Celtic an almost impossible task at Murrayfield and goals in either half from Michal Zyro and Michal Kucharczyk on Wednesday night consigned them to the final qualification phase of the Europa League.
Celtic will be seeded for the competition when the draw is made on Friday but Walker insists the Champions League is the only place Hoops fans want to see their team.
Speaking to Sky Sports News he said: "Last season they were drawn against Barcelona, AC Milan and Ajax. They struggled against all three and the supporters knew then that the team had to be strengthened.
"Instead of that Celtic have got rid of a big European player in Georgios Samaras and failed to make any sort of effort to replace him. They have also failed to support the new manager.
"Obviously he is due a bit of criticism, you cannot escape that as a new manager, but I just don't think he was helped by the lack of support he has had from the board. Celtic needed to strengthen and they have failed to do so at a vital time.
"When you are used to Champions League football, the Europa League - if they get there at all - is a very poor substitute."
And Walker admits concern over the lack of activity in the transfer market since Deila was drafted in as Neil Lennon's replacement in early June.
Goalkeeper Craig Gordon, who has not played a competitive game for almost two years, was signed on a free transfer while Cardiff attacking midfielder Jo Inge Berget has joined on a six-month loan deal.
"I understand it is very difficult to attract quality players but I don't even see or hear of Celtic making any sort of effort to get players in, players that they badly need," said Walker.
"Clearly Celtic need to spend a bit of money that they have and try and get some players in with a bit of excitement just to lift the supporters.
"They know they will watch a team who will win the title this season but I think when Ronny Deila came in he said they would play with energy and drive and I think he needs new players to try and achieve that.
"Last night was a massive blow to them, but when you look at it over the two legs you could see Celtic went out with a whimper.
"The Celtic support expected a lot better from the players, a bit of sustained pressure and plenty of efforts on goal but they really saw very little and you could see, feel and hear the anger in the stadium which was directed not only to the manager but the board and chief executive.
"The fans know they have lost the showpiece occasions of the season. Celtic supporters have lost the opportunity to show the rest of Europe how colourful and how supportive they can be of their team on Champions League nights at Celtic Park."
While Walker sympathises with Deila on the lack of backing from the board, he admits the Norwegian's honeymoon period could be very short if the fans fail to see an improvement on the park over the coming weeks.
"Supporters will question whether he is the right man but I am absolutely convinced he will be given time," he said.
"Celtic will win the league again and need to improve on their showing in the domestic cups from last season. But I think it will be the style of their play that the supporters will be interested in looking at over the next few weeks and months.
"If Celtic can play with a bit of verve, pace and passion and excite the fans then I'm sure Ronny Deila will be welcomed with open arms.
"But when you lose two big games in the manner he has then absolutely you have to accept any criticism that will be coming your way."
Watch Celtic kick off their Premiership campaign against St Johnstone from McDiarmid Park live on Sky Sports 5 from 7.30pm next Wednesday