Andy Walker claims Neil Lennon's tactics and transfer policy must be called into question following Celtic's shock Champions League exit.
Romanian champions CFR Cluj knocked Celtic out of the Champions League in the third qualifying round after the Hoops lost 4-3 at home in the second leg on Tuesday night, crashing out 5-4 on aggregate, meaning for the fourth time in the last six years Celtic champions have failed to qualify for the Champions League group stage.
Former Celtic striker Walker is critical of the decisions to play Olivier Ntcham, who reportedly wants to quit Scottish football, while deploying midfielder Callum McGregor at left-back and believes both played a major role in the defeat to Cluj.
Speaking to Sky Sports News, Walker said: "I'm sure Lennon will be happy to accept his part of the blame. To play McGregor at left-back, that's happened before and it didn't work.
"You are taking arguably Celtic's most creative player out of midfield in favour of someone like Ntcham, who said only very recently that he doesn't want to be at the club, and that he's too good for Scottish football.
"The team last night has a lot of talent from middle to front but McGregor should be in the middle of the park. Defensively Celtic need strengthening.
"Celtic have lost at home to Bayern Munich and PSG but to lose four goals against an outfit like Cluj I think that tells you Celtic weren't prepared as well as they should have been and that's down to the board and down to the team selection from the manager."
New signings left on the bench
Lennon also opted to leave new £7m centre-half Christopher Jullien, and £3m left-back Boli Bolingoli, on the bench, much to the puzzlement of Walker.
"To spend £10m on two defensive players and then have them on the bench, you either have to question whether they are good enough for the most important games of the season," Walker added. "Or whether the manager doesn't fancy them and that then begs the question of who did sign them?
"Steven Gerrard at Rangers would love to have the ability to spend £7m on one player or even £3m on one player, so to have those two players on the bench in inexplicable.
"The fact that this defeat will impact on the spending that Celtic will make is baffling because it is a club that are very much cash-rich but they prefer to have money in the bank rather that seeing it on the pitch.
"Celtic are not the sort of team to go out and give the supporters the best possible chance of European football. They have been run very well and bring in players at a low price and sell them on, but they don't invest in the most important part of the season and leave it far too late.
"There is no reason that it should impact on Lennon's spending but it will, and that is what is frustrating Celtic supporters. They (the board) just seem content with trying just to be the dominant side in Scotland."
Lennon: Only ourselves to blame
Lennon speaking in his post-match press conference said: "We all wanted Champions League football. We've only got ourselves to blame for not doing that. We've been loose and switched off at the important moments of the game.
"The players are bitterly disappointed. They've all missed out on a good opportunity, we all have. We are all collectively responsible.
"We had enough quality out there to win both ties. We had good chances and didn't take them, they would have put the game beyond Cluj."