On May 11, 2011, I got my first in-person look at Zack Wheeler. He's still a little rough around the edges, but as far as present stuff goes, he's one of the best pitchers I've ever seen in the California League. I've created gifs of each of his pitches. I've also embedded a condensed video of his entire start at the very bottom of this article.
Fastball
Wheeler works off a 91-94 MPH fastball with hard, sharp-bending tail. He doesn't nibble with the pitch and likes pitching inside. Able to touch 95 MPH when he needs it, he also was willing to slow things down and sacrifice velocity for command when he needed it -- dipped to the high-80s at times.
His fastball command came and went a little when I saw him. The pitch also straightened out at times. Wheeler exhibited better fastball command when pitching inside. He struggled with consistently throwing strikes on the outside corner. He was also up in the zone with his fastball a bit too much, something that could easily lead to lower ground-ball rates as he advances.
Because of his excellent movement and velocity, Wheeler's fastball projects as a plus offering. He attacks the zone with a lot of quality strikes and isn't at all afraid to challenge hitters. When he allows walks, it's not because he's nibbling, it's because he can't hit his spots.
Curveball
Wheeler's 72-78 MPH curveball has elite, two-plane break and is a no-doubt swing-and-miss offering. The gif to the right is of him buckling Jose Altuve on an 0-1 count. He went on to strike Altuve out with another curveball -- this time in the dirt -- the very next pitch.
Wheeler threw mainly curveballs in his third inning of work, after only occasionally showing the pitch his first two innings. He consistently snapped of curveballs like the one in the gif. It was very impressive. When he located it where he wanted to, it was unfair. Hitters could know it was coming and still swing through it.
He generally kept the pitch down in the zone, but he missed in the dirt a bit too much. Similar to his fastball, Wheeler has room to improve his curveball command. He's presently getting some swing-throughs on curveballs out of the zone that many big leaguer hitters won't offer at.
Still, the pitch is already above-average and it's close to surfacing as a true plus offering. As impressive as Wheeler's fastball was, I think his curveball is his best offering.
Changeup
Wheeler lowered his arm slot a little when he threw his 82-84 MPH changeup. It had bit of sink and late tail to it, but not as much as his fastball. Given how infrequently he threw it, I wasn't able to get a great read on it. I liked the movement, though he's going to need to up his arm speed and intent as well as his arm slot up a little in order to be able to sell it better to big-league hitters.
The changeup I saw was a below-average offering, but it's not far from being average and it could end up being slightly above.
Mechanics/Misc.
Wheeler throws from a 3/4 arm slot. He has a good, aggressive tempo. He's not a max effort pitcher. He stays balanced through his delivery and repeats it well.
He has big feet and broad shoulders. There's a good chance that his body isn't done filling out. Added muscle could lead to better body control, better command and more velocity.
That said, Wheeler has a big red flag in his mechanics, a timing problem. He doesn't begin to turn his forearm over and get the ball into the driveline until after he plants his front foot, despite his long stride. This leads to halted lower body momentum and a lot of stress on his throwing arm. Ideally, a pitcher will generate energy with his core then bring his arm along for the ride. Though Wheeler looks balanced and his velocity appears easy, he doesn't create an efficient 
chain of kinetic energy in his delivery. His arm is left to do a lot of the work.
Jarrod Parker and Jeremy Hellickson, who have both missed time with arm injuries -- Parker had Tommy John surgery in 2009 and Hellickson has been sidelined with multiple arm injuries, though he hasn't yet required surgery -- have shown the same inefficiency.
Mechanical inefficiencies can be very difficult to spot at game speed, especially with someone who has as quick of an arm as Wheeler, so I've included a frame from my video to the right. Notice how his front foot is planted and his forearm is still nearly horizontal to the ground. He has halted his lower body before getting the ball up into the driveline and letting the energy he has built up through his core allow his arm to come along for the ride as he throws the pitch.
Now this doesn't spell certain doom. There are a lot of variables when it comes to pitcher health, many of which still have yet to be understood. But I view Wheeler as a above-average injury risk because of it.
That said, I wouldn't change a thing about his mechanics at this stage in the game. He may be less from a year away from contributing at the big-league level. A few more healthy seasons and Wheeler could easily prove an outstanding investment for the Giants. And pitchers have strung together elite careers with less-inspiring mechanics than Wheeler.
Overall
Wheeler has two pitches that could surface as plus, in his fastball and curveball. The changeup I saw doesn't look like it will be more than a slightly above-average pitch. He could use more seasoning with his changeup as well as his command, but he could be a back-of-the-rotation starter in the big leagues tomorrow. His upside, which he could still be 3-4 years from reaching, is as a very good No. 2 starter and maybe even an ace for stretches of time.
Every young pitcher comes with a good amount of injury risk, but Wheeler's mechanics make him an unlikely bet to avoid major arm injuries over the next five years. Hopefully he can stay healthy, mature into his frame, sharpen his command and get a chance to face big-league hitters. Wheeler is a treat to watch and one of the higher upside arms in the minors.
Follow Adam on Twitter @AdamWFoster.