From what I hear, this was either (A) extreme caution caused by very minor muscle soreness, (B) they're a little worried since Jered sat out all of last year (his agent was Boras), and that maybe he shouldn't hit 200 innings this year and the added time off with the all star break meant a very long rest while just missing one start, (C) They needed an excuse to showcase the two starters they brought up from AAA before the trade deadline, or (D) some combination of all the above.
Nobody seems worried at all about Jered. The rest was reportedly against his wishes too. He wanted to pitch. For what it's worth, his last start, he was coasting from beginning to end, didn't look like a guy in any kind of pain. I have him on my team, and if anything because of his age, I don't mind the fact that he got a LONG midseason break before the final push to the end.