I have very mixed feelings about labeling players a 'injury-prone.' There is very little scientific data to suggest that certain players suddenly develop an affinity to being injured that is not due purely to random chance that is unpredictable, unless it is due to significant underlying medical issues. That said, it certainly is true that some people's bodies age less well than others, some people's bones are more durable than others, etc. I always have trouble with the idea that 'injury-proneness' is an attribute that can be quantified and predicted. As such, using it to justify undervaluing Saunders is hard for me.
That being said, players like Saunders and Gutierrez have such a proclivity for injuries, including these weird fluke ones, that it does genuinely feel like it defies logic and reason.
While I don't think you can use an injury like that to retrospectively justify the trade, and I still think we probably didn't get as much as Saunders could've been worth, the Mariners' decision to cut bait turned out to be a stroke of good luck for an organization that badly needs it. Just like in poker, sometimes you don't make the play that has the highest probability of success, but it's your gut and you go with it and you win. More often than not, going with your 'gut' against the odds is not the right choice.
Sometimes you get lucky. The M's got lucky. Sadly, Saunders didn't. Hope he recovers.