Per John Blake and the rest of the Twitterati. Jeff Wilson says that Darvish experienced stiffness in his neck this morning and it didn't dissipate after treatment, and the official word on Darvish's next start is TBA.
I assume this isn't a serious issue and that Darvish will be okay to go very soon (perhaps within the next day or two), but this could complicate matters a bit as far as Texas clinching the AL West as soon as possible and being able to align its post-season rotation to its liking.
This news is also a bummer because we were expecting to see Yu Darvish and we're getting Scott Feldman instead. That doesn't require any further explanation.