The Blues have won two of their first three matches this season, but were beaten 2-0 by Liverpool in the Premier League last weekend, with Andreas Christensen shown a straight red card.
There is little shame in losing to the champions, but Keown believes the pairing of Christensen and Kurt Zouma, which Lampard has favoured so far, is not the answer in central defence.
Summer signing Thiago Silva is being eased into the team, making his debut in the Carabao Cup win over Barnsley on Wednesday, but Rudiger has been left out entirely so far this campaign.
The German has not featured in the Premier League or Carabao Cup, but Keown believes he is the best option Lampard has in the middle of defence and he needs to pick him rather than sell him, which looks more likely at the moment.
‘The goals they concede…they don’t seem to know how to mark properly from crosses,’ Keown told talkSPORT. ‘Their defence is a disgrace.
‘Silva comes in, maybe he resolves that. Maybe Rudiger actually is the best defender and he will perform better with somebody alongside him who will tell him how to craft, how to mark from crosses.
‘There are elements of their team at the back that are not quite right, defending’s gone out the window, by the way, in the Premier League at the moment.
‘I think Rudiger [should play], I know he has problems from set-pieces, but they all do.
‘Zouma’s maybe a better header of the ball but Zouma’s prone to mistakes.
‘I think Rudiger’s the best defender, tie him down with Silva, that is the long-term partnership, for me.
‘But it looks like Rudiger’s going out the door, so it’s all opinions.’
Barcelona, Roma and Paris Saint-Germain have all been linked with a move for the Germany international, who is unlikely to want to stay at Stamford Bridge as fifth-choice centre-back.
When asked about the futures of both Rudiger and Jorginho, Lampard said: ‘I don’t expect anything in the window, they are our players.
‘There are lots of rumours, not just on those players but on a lot of our players.
‘Every player will always be a case-by-case situation that has to be a happy solution for the player, myself and for the club.
‘We have a big squad, we know that, but at the moment players remain ours and we will see as time goes on.’