It's a shame this happened. It stinks for Scherzer. Was the ball off the plate? Yeah it was. Should the ump have called HBP, probably. When I watch the video in slow motion, if Tabata had simply left his left arm where it was, the pitch would have sailed 6 inches under it. There is no question he dipped his elbow in order to be hit by the ball. Would he have done it without the elbow guard on? I kind of doubt it. Getting hit on the tip of the elbow would have hurt like crazy. To be honest, looking at it closely, I don't think the ball would have even hit him in the elbow if not for the guard. The guard comes down below the elbow and the pitch just barely nicked that.
Does this situation call for a rules change? I don't think so. It's just one of those things. If Scherzer does not lose that pitch inside, he most likely has a perfect game (you never know whether Tabata would have gotten a hit if he didn't get hit). If Tabata doesn't dip that elbow into the pitch, he may have a perfect game.
All these things are the reason there have only been 22 Perfect Games thrown in the history of professional baseball. It is the hardest thing to do for a reason. It strikes us all that much harder because he lost this one with 2 outs and 2 strikes on the batter in the 9th inning. Ohhhhhh so close.