No question, we need another body to go with Evans, Jones, Smalling and Blackett solely because of the past history of injuries for 3 of the 4. Who that body is remains difficult.
We established on this forum the lack of top defenders in world football right now. The likelihood of us signing someone like Hummels is incredibly slim, so wouldn't it make more sense to develop our own players rather than looking to buy a top one that we probably can't get anyway? It's not like Jones, Smalling and Evans don't have experience against the top teams we will face this season in the league. As for Blackett, I'd venture to say that Liverpool in a friendly will give us a tougher game than alot of teams in the League this season.
I think we can go cheap on this option and buy a Vermaelen or Vlaar to use for cover. It would be in our best interests long term to try to use this season to establish Jones and Smalling as top centerbacks.