Keith Law at ESPN:
A scout I spoke to who saw him recently said that Wheeler's curveball, once a plus pitch in my view, was just average for him as Wheeler was throwing both a curveball and a slider and seemed to be caught between the two of them.The thing is that Wheeler's two breaking balls both have their uses. I do not buy that he should focus only on one.
He still has plenty to recommend him, as he works with plus velocity and fills up the strike zone, with just 12 walks in his past eight starts, but I'd like to see him settle on just one breaking ball so he can miss bats in the majors the way he has done in the minors so far.
Mark Anderson and Bret Sayre at Baseball Prospectus are much more positive about Wheeler's breaking pitches:
His bread and butter is a mid-90s fastball that can reach 97-98 without losing the natural life that makes it a true plus-plus offering. His fastball jumps out of his hand and can explode in on right-handed hitters. Wheeler has struggled with maintaining his angle and working low in the zone in the past but he made significant strides in that regard this year.
His curveball is the feature secondary pitch sitting in the low- to mid-70s with tight rotation and hard break. After years of scouts projecting his curveball out to plus-plus levels, Wheeler finally put it together this year with consistent 70-grade hammers that buckled the knees of PCL hitters. His slider is a harder offering, sitting in the upper-80s and featuring multiple looks. He can vary the sharpness and tilt of the pitch without sacrificing velocity, giving him an unpredictable weapon that marries well with the rest of his power arsenal.
At Fangraphs, Marc Hulet saw Wheeler in Fresno on June 1. Wheeler's line that day: 6 IP, 3 H, 1 R, 1 ER, 1 BB, 6 K.
He flashed his mid-90s velocity and two breaking balls but his changeup was seldom used, which disappointed me given that it was identified as something he needed to work on in 2013. The first curveball he threw during the game was quite possibly his best and he struggled to command both his curve and fastball throughout the contest. He telegraphed the curveball early in the game and slowed his arm down but it was less noticeable as the game progressed. His slider was average but I’d like to see a tighter break to it.Emphasis added.
Wheeler’s fastball showed good velocity but his command of the pitch was average-at-best.
I expect him to be susceptible to the home runs at the big league level because of his lack of consistent fastball command in the upper half of the strike zone.