Rodgers on Can:
"He is a multi-functional player that can play in any of those three midfield positions. And he played at centre-half at Bayer Leverkusen and played in the Champions League at left-back, so he's a footballer who will do very well for us."
So BR loves the fact EC can play multiple positions - he most definitely signed him because of his flexibility - but didn't sign him as an out and out CB. Keni said it above, you can't play Can as a CB when you have actual CBs fit and able to play, it would destroy team unity, never mind massive loss of confidence, which is the one thing this team needs more than anything else at present.
BR was almost certainly only mentioning EC can play at CB/LB as a last resort, in the event of an emergency (as in multiple CBs/LBs injured or in actual play when all our subs are used up but Can is on the pitch when a CB has to leave the field due to a Red Card or injury) !