Common sense is knowing your personnel and knowing what, if anything, is wrong and what their breaking point is.
Dusty is far more equipped to knowing what is wrong with Chapman and when to pull him than fans sitting in front of a computer reading radar graphics.
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The thing that really worries you about Chapman is this is three times in a row the velocity is down. If it was one night you could maybe say he couldn't get loose in the cooler weather or something like that.
As far as examples, there are decades worth of baseball at one's disposal to know that pitchers go through periods where their velocity will fluctuate. It's not a static thing. It's not common for it to drop suddenly over the course of a large sample, but over a few games... that's not really uncommon.
Wile we're debating Chapman's velocity, Hanigan just had a heck of an at bat.