Sportsfan - sure, he's not the only pitcher to lose velocity over the season (some go up), but your suggestion that lots of pitchers wear down over the season kind of proves the shutdown point. The point being, it's tired pitchers that have the greatest risk of injury. If he's showing evidence of tiring, being young and coming off an injury, all that points to being cautious and treating these at least as yellow lights.
and by yellow lights, I don't mean speed up before they turn red.