I'm not sure if you can call a tough match at Newcastle a must-win right now, but this weekend's trip for Aston Villa is certainly a "dear god it would be amazing to win" match. If the unlikely is to happen, it will have to be in spite of a massive rash of defensive injuries that will see Kieran Richardson definitely out, and may well sideline Alan Hutton and Aly Cissokho as well.
Richardson picked up a knock in the first half of last weekend's match that forced him off then. Cissokho and Hutton are both nursing injuries. The former is doubtful for the weekend and the latter will face a late fitness test. Combine the walking dead with the fact that Philippe Senderos is still out and Ron Vlaar will miss this match due to a suspension and you've got a recipe for problems.
If we assume that none of the above-mentioned players see the pitch two options arise. At right back, Villa have quite a bit of depth, as either Leandro Bacuna or Matthew Lowton could step in. On the other side of the pitch we've got... Ciaran Clark? Fabian Delph? Both have filled in at the position before. I'd rather see Clark in the slot, but that means that there would be absolutely no depth at centre back where we'd see Nathan Baker and Jores Okore.
The other option is to start with three central defenders and forego the right and left backs. This would likely result in something like a 3-4-3 or 3-5-2 formation. There is the issue of no centre back depth, but you could plan to use midfielders who can slot back in defense if need be. For instance, start Delph and Bacuna alongside three other midfielders. Then, if one of Baker, Clark, or Okore had to come off, you could move Delph and Bacuna back to left and right backs, respectively, and run with a (slightly less than optimal) 4-3-3.
Regardless, if all of the injuries end up keeping players off the pitch, life will certainly be difficult for Tim Sherwood. What do you think the gaffer ought to do?