[quote=@Jbcool]Marines have taken far more than being smashed in the head by an Ork and survived.[/quote]

Well, he didn't Just take a blow to the head from an ork, that was just the most recent. As I recall, he's fallen several stories only to be impaled on a large spike, in addition to taking a large volley of bolter fire. I feel like there was more, for some reason, but can't find it.
Anyway, my point is, basically any of those things would outright kill a normal human, several times over. More importantly, though, the extra organs marines are implanted with are fully capable of being damaged to the point of needing repair. Being as most are situated in the chest, and that's where the spike hit him, there's plenty of room to allow for the need of surgery. That a trained apothecary stated he'd be dead without the appropriate care acts to confirm that he needs it.
Marines are tough, but Xeph has taken the kind of damage that could put one down. In addition, he's a techmarine, so it's entirely possible his mechanical enhancements were damaged in the process, and That's why he's dying.

I'm not saying it's Impossible for a healing coma to pull him through, but the Sus-an membrane has its limitations. The most important of which is that, if he Did enter the coma, he'd need an apothecary to wake him up anyway. Plus, how long it would take him to heal those wounds is entirely uncertain. That is, of course, assuming the Sus-an wasn't damaged to the point of being inoperational by either the fall or the bolter fire, which can happen. The fact that he's still bleeding means that at least one of the organs which are responsible for preventing that has also been damaged, which manatee indirectly addressed in his post.

Marines wear armor for a reason. Yea, compared to a human they can take a lot, but that's really not saying much. That's why I don't go by the books, they make marines out to be a lot more than they are. The truth is, space marines die all the time. That's why every chapter is Constantly training new recruits, despite the 1,000 marine limit stated in the Codex Astartes. In the grand scheme of things, in a world where standard-issue weapons are literally automatic mini-RPG launchers, they aren't the nigh invulnerable demi-gods the books act like they are.