Caldwell is far and away the best central defender we have. He's captaining his side in the EPL. I don't see anyone close to that level. Not Berra, not Hanley, not Webster, not Mulgrew. I'm not saying he's world class but he's the best we've got. Who else is there capable of playing at centre-back in the EPL?
Adam's another that I being criticised but, again, he's the only player we've got that can pass a ball. D.Fletcher, Brown, Morrison and Dorrans are all good hard-running players...but they can't pass a ball. Unless we play dogged defensive or long-ball, we need someone that can pass a ball 10 yards without screwing up. I don't see many contenders.
We've got to realise our limitations but replacing one player with an even worse player won't get us too far.