O.K. Here's my take on it. Acuna learned two valuable lessons here. First, when you go on a home run hitting rampage, especially against one team, you're going to make opposing pitchers irate. Said pitchers will compensate with the 'Brush back' pitch, and when that doesn't work, you get plunked... PLAN ON IT!!! That's the way baseball has worked for that last 60 years that I remember. Secondly, he learned that when getting plunked is inevitable, don't try to block it with your arm/elbow!!! Lift your arms, tighten up and take one for the team,,, remember, it is part of the game.
Thus endith the lessons.
Now, for those of you who think it should not be part of the game. There are specific rules regarding the plunk, and the umpires followed them. It is the batters responsibility to protect him/herself when in the batters box. What else do you want, chicken wire cages, like Red Neck honky-tonks??? Lighten up, it's part of the game, and don't come back with, "Well, it shouldn't be!", because I don't want Baseball watered down. It is a dangerous game and the players know it when they take the field, specially at the highest level!
Thus endith the rant.