According to my very sophisticated research (looking at wiki and sofifa for EAFC ratings), Lilian Brassier is the only other left footed CB. They recently signed him on loan with option to buy from Brest. He played 30 matches in Ligue 1 for Brest so he will be good competition for Derek for sure. They play 5 at the back, so he could theoretically fill in at LCB or CB if he doesn't win a starting spot. I don't think it would be too far-fetched for him to start though.