John Collins says Celtic felt the loss of Victor Wanyama and Gary Hooper as they slipped to a heavy defeat at home to AC Milan and saw their Champions League knockout hopes dashed.
Neil Lennon's men were beaten 3-0 by the Italians on Tuesday evening with goals from Kaka, Christian Zapata and Mario Balotelli doing the damage, leaving the Bhoys bottom of Group H and unable to reach the last 16.
Collins watched the game in the Sky Sports studio and says it was clear that the Hoops missed the goal threat of Hooper and midfield strength of Wanyama, who joined Premier League sides Norwich and Southampton respectively over the summer.
"Celtic lost three key players in Wanyama, [centre-back Kelvin] Wilson, and Hooper, and while [Virgil] Van Dijk has fitted in no problem at the back, they haven't replaced Wanyama and Hooper and doing that with £2.5-3million is difficult," said Collins.
"The reality is that if you sell your best players it is going to be hard to go against smart players in the Champions League, as it's not the Scottish Premiership where you are playing against average players and can get away with mistakes."
Celtic skipper Scott Brown was absent for Celtic against the Rossoneri after serving the final match of his European suspension for kicking out at Barcelona star Neymar in the 1-0 defeat to the Catalans in October.
Collins feels the Bhoys would have coped better with the threat of Balotelli and his team-mates had their 28-year-old captain been available.
But he says the Glaswegians did not aid their cause with some shoddy defending at corners, which allowed Kaka and Zapata to put the Milanese two up.
"Brown is a huge player for Celtic and has been a huge miss in the middle of the park because he can get hold of the ball and keep it, win tackles and get about the pitch," added the former Bhoy.
"I have no doubt Celtic would have been more solid with him in the team, but he has got to learn that, as club captain, he cannot do silly, petty things - and only he, not his manager or team-mates, can make him realise that.
"Celtic had chances and in general play had a good part of the game - there was not three goals in it - but they went to sleep at two set-pieces.
"They were static and their marking was horrendous - they had plenty of bodies in there but no-one was picking anyone up - and that cost them dear."
For more Champions League, news, views and opinion, hit this link.