let's not even compare Petko and Alex because the stories of the two and the actual abilities are completely differnt
Andrea is a player that reached much more than her potential thanks to hard work and hard work only, most of her injuries were due to the same reason: over-working. She was winning matches only thanks to her physical shape and everything reguarding her is built, so it's expected that sooner or later she would start getting injured.
Alex on the other hand has a lot of talent and her physical injuries are not due to over-working, but other issues and a fair bit of bad luck...she's the kind of player that if she's retiring today would have achieved much less than her talent might have given her (though 2 premier titles are something huge).
As Aaron said though, I wouldn't be as negative, I am sure she only needs time and confidence