They don't - also we have had guys out for way longer then most teams deal with.
The bigger factor is most team's in this league have at least 3 MLS quality CB's. That way a team who suffers injuries to their top 2 CB's can make do until one of those 2 gets back.
We thought we had a decent #3 in Hagglund or Zavs or VDW. VDW has been injured while Zavs & Hagglund have regressed.