Quite a lot of posters realise that they all have enough talent or are good enough, if they would stay injury free. They don't though and that's reason enough to look for replacements. To answer your question, our team won't be able to build a solid relationship at the back, since they are or won't be available for a prolonged period of time, at least the past suggests as much. Van Gaal will be aware of their injury records and he probably decided to trust them to turn it around, since he was confident of his own capabilities as a coach, he probably was wrong and will react to that. With all of that being said, the unlikely might happen and they turn it around, but I don't believe it anymore. Ferguson put his trust in Rafael, Smalling, Jones and Evans, for quite some time I did too and still rate all of them highly, still any sympathy I had for them is completely gone this season. They could've have stepped up since Ferguson retired, but since then they actually look worse, bar Rafael who has improved again under van Gaal, I don't see any reason not to look at potential alternatives. Van Gaal also did Smalling and Jones a favour in not playing them at right-back anymore unlike his predecessors. Obviously the change of training regime might have caused injuries to them as well, but it's always them. In my opinion it's completely normal and justified that supporters are beginning or already have lost all their faith in them or one of them.
I agree that Vlaar isn't the solution and Vertonghen simply doesn't convince me. Unlike others I've actually liked Rojo's performances so far given the circumstances.