Jenrry Mejia's approach last night was consistent with what you would expect from a 20 year-old making his major league debut. 13 pitches, 11 of which were fastballs. His two-seamer and four-seamer were around 97 mph. The changeups he threw were around 89-90 mph, also known as John Maine's fastball velocity. Mejia wasn't exactly painting corners, which is one reason why the Marlins made such solid contact against him. Given his somewhat shaky control in the minor leagues, it's possible he was simply trying to throw strikes at the expense of hitting spots:
(This is from the catcher's point of view. FF=4-seam fastball; FT=2-seam fastball; CH=changeup; image via Texas Leaguers)
The horizontal movement on his fastball decreased compared to a performance in the AFL that Sam looked at last fall. But as with any early season, small-sample-size performance, there's no reason to draw any conclusions here. What can be said is that if Mejia brings nothing but straight heat to major league hitters, he will probably get knocked around a bit. Ideally he should be in a situation conducive to working on his changeup and curveball, which will help him develop into.... ah, forget it, what's the point.