A backup LB is really a weird issue. In theory, we have Adriano, Montoya and (let's pretend for a moment) Abidal. But Abidal is not match ready, Adriano is injured most of the time, which brings us to situations like this, where we have no backups when either Alba or Alves is injured. We bought Alba because we needed someone to take Abi's place. But if Alba can't play, we're back at square one.
What we need is another solid option who can play left back and centre back. That's what Abidal always provided and is what we've missed.