Kris Commons reckons Celtic must sign a new central defender in the summer if they are to progress in Europe next season.
The former Parkhead winger's signing demand comes after being left unimpressed by fringe players in the defeat to Rangers at Ibrox.
Commons - in his Daily Mail column - insisted "several players" did themselves "no favours at all" with their displays against Rangers.
Celtic had a much-changed rearguard for the fixture with Alexandro Bernabei, Anthony Ralston and Yuki Kobayashi all starting alongside Carl Starfelt.
Greg Taylor was on the bench for the fixture with Cameron Carter-Vickers and Alistair Johnston out injured.
Now, Commons has claimed Celtic dearly missed Carter-Vickers as he called for defensive reinforcements to be signed in the post-season.
He wrote: "If Celtic want to progress in Europe, I still think there's a slight weakness in one of the centre-backs that could be improved and upgraded. But let's not pretend Starfelt was alone in that regard on Saturday. There were several players in the Celtic team who did themselves no favours at all.
"Alexandro Bernabei has been a disappointing signing thus far. He was supposed to be the new first-choice left-back ahead of Greg Taylor. Celtic paid £3.75million to sign him from Argentina last summer, but it's Taylor who has ended up having an outstanding season and has rightly kept Bernabei out the team.
"Yuki Kobayashi is neat and tidy on the ball. However, the pace and physicality of the game looked too much for him at times on Saturday. He certainly didn't offer the same level of authority and assurance as Carter-Vickers does when he plays.
"I reckon Carter-Vickers is actually the most dominant defender we've seen in Scottish football since Van Dijk. So it's no surprise Celtic should look so vulnerable when he doesn't play. He's a massive loss."
However, Commons insists the defeat to Rangers doesn't change much for Celtic heading into the new season as he suggested Postecoglou's summer should be viewed as "more a case of evolution than revolution".
He added: "I don't think it changes a great deal for next season. Rangers still face a massive summer rebuild. Celtic will still start next season as red-hot favourites to win a third league title on the bounce. Rangers have at least shown that they can lay a glove on their rivals and at least make it a proper title fight next time round.
"My old team won't face anywhere near the same level of transfer activity as Rangers in the summer. For Postecoglou, it should be more a case of evolution than revolution as he looks to tweak one or two areas in his squad."
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