Former Celtic midfielder Murdo MacLeod feels poor recruitment has been a major factor in their slump in form.
Manager Neil Lennon remains under intense pressure from supporters after Celtic slipped 13 points behind Scottish Premiership leaders Rangers on Sunday.
Celtic followed three consecutive defeats with a home draw against St Johnstone and have only taken six points from their last five league games.
MacLeod believes players must share the blame with Lennon for the dramatic reversal in fortunes – which has seen them win just twice in 12 matches – but he also questioned the summer signings.
Left-back Diego Laxalt has settled in well but goalkeeper Vasilis Barkas has been in and out of the team, defender Shane Duffy has struggled and has been out of the starting line-up for six games, 21-year-old midfielder David Turnbull has only started once and striker Albian Ajeti has not scored since September.
MacLeod, who played for Celtic for nine years, told Sky Sports News: "Neil is the one that's been getting all the stick, but the players have got to look at themselves – every single one of them – and just look in the mirror: 'What am I not doing that I used to do last season?'
"Even a lot of the new players, because I always felt when you are bringing new players into the club, they have got to be better than the players that's already at the club. And I think this season, that's not happened."
Lennon was publicly backed by the club last week following a Betfred Cup defeat by Ross County and looks set to continue in his role.
Already-eliminated Celtic host Europa League Group H leaders Lille on Thursday before welcoming Kilmarnock to Parkhead on Sunday.
Their subsequent game is last season's delayed William Hill Scottish Cup final against Hearts.
MacLeod said: "You just felt after the Ross County game something was going to change and then after the game on Sunday, St Johnstone at home, you've got to win those games and it's not happening just now.
"Neil can win one game then he looks for another game to win and if he gets two or three in a run, then everything is back to normal. But you can't let the gap be as big as it is just now.
"Of course if you're still at the club and you win the Scottish Cup, you can't sack a manager who has just won the Scottish Cup. It's got to keep on going. So hopefully by then, Celtic are on a run."
MacLeod was assistant to Wim Jansen when Celtic stopped Rangers winning a record 10th-consecutive title in 1998 and the Ibrox side are well on their way to preventing the Hoops reaching that mark.
"They have been unbelievable," MacLeod said. "They are scoring goals, they are very fit looking, they are sharp. Everything is very positive about them.
"Even in the European tie during the week there, they didn't play as well but they still came away with a victory. That says an awful lot about a team when they are struggling a little but they still win."
But MacLeod said of Celtic's title quest: "There's still a chance, you can't just put the boots away and that's you.
"You just have to get on with it and hope it's going to start turning at some point."
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