Antonio Rudiger is Chelsea's "best natural defender", according to Tony Cascarino, who thinks Fikayo Tomori and Kurt Zouma have struggled to convince in his absence.
Chelsea's five-match winning streak in the Premier League came to an end on Saturday, as they suffered a 2-1 defeat to Manchester City at the Etihad Stadium.
The Blues started the game well and took the lead through N'Golo Kante, before lapses in concentration at the back allowed the reigning champions to get back into the game.
Goals from Kevin De Bruyne and Riyad Mahrez shortly before half-time ensured City moved up to third, while Frank Lampard's men dropped into fourth, 11 points behind leaders Liverpool.
Chelsea have now conceded 19 goals in 13 Premier League fixtures, more than any other side in the top six - a record which Cascarino attributes to a Rudiger-shaped hole in the defence.
The former Blues forward has not been encouraged by Tomori and Zouma's partnership in recent weeks, as he told TalkSPORT: "I think there’s a lot of learning to be done with Tomori and Zouma.
“They’re a partnership that Frank trusts, but there are mistakes there too regularly.
“They’re both young men in the early part of their career, but big clubs don’t allow you to go through this period of indifference.
“They miss Rudiger enormously, because he is their best natural defender.”