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Rob Johnson is dropping pitches behind the palte because he is trying too hard to frame them for the pitcher. In both instances tonight, the ball hit off the webbing of his glove and rolled away because he was keeing his hand as still as possible to convince the ump that the pitcher was hitting the mitt. Have you ever seen the movement on a Felix Hernandez fastball? He's underestimating the lateral movement on that ball.
I agree on Langerhans' swing being too whippy though...he seems to be a good guy to use against soft tossing pitchers (because his swing is longer and he prefers to drive the ball the opposite way anyway) but he won't do well against the power pitchers.