However, his ability to throw strikes, keep the ball on the ground and get lefties out makes him the most polished pitching prospect in the system. His upside isn't that of an ace, but he could figure into the equation at the back of the Giants' rotation as soon as this season.
In four minor league seasons since the Giants drafted him in the 12th round of the 2009 draft out of East Carolina, Heston has put up a 3.13 overall ERA while striking out 7.9 hitters per nine innings against just 2.3 walks. He's also only allowed 459 hits and 18 home runs over 483.1 minor league innings.
Bochy said of Heston, "He commands the ball well and has good sink on it. I know it's just [throwing in the] bullpen, but this is his first big league camp."
Posey was equally impressed, saying, "He had good sink and run on his fastball to both sides of the plate, and he commanded his other pitches really well."
Bochy also told Andrew Baggarly of CSN Bay Area, “I like his delivery. It looks like he commands the ball well, and it’s moving well. It looks like he knows what he wants to do.”
While Heston's minor league numbers are impressive, his lack of pure stuff could hold him back. Former Giants' prospect Joe Martinez was a similar type of ground-ball pitcher who succeeded without elite stuff in the minor leagues. However, his lack of velocity has caught up to him in the big leagues, where he's put up a 6.22 ERA over parts of three seasons.
With a continued strong showing this spring, Heston could be the first man up from the minor leagues if one of the starting five goes down at some point this season. He'll have to bring elite command and movement in order to succeed in the major leagues as a right-handed pitcher without plus velocity.
Another high-upside starting-pitching prospect in camp is lefty Michael Kickham. Kickham has better velocity than Heston, sitting in the low-90's with his fastball. He was also able to hold right-handed hitters in check last year, as righties hit only .221 against him. Additionally, he allowed just eight home runs in 150.2 innings of work.
Kickham's biggest weakness is his lack of control. He walked 75 hitters last season and has averaged 3.9 BB/9 so far in his minor league career.
Yet with his plus velocity, strikeout stuff, effectiveness against righties and ability to keep the ball in the park, Kickham could be a rotation candidate as soon as 2014. He has the stuff to pitch in the middle of a big league rotation, but his control will determine if he reaches that goal.
Edwin Escobar is another high-upside lefty, but he's not as close to the big leagues as Kickham or Heston. The 20-year-old put it all together last year at Low-A Augusta—putting up a 2.96 ERA with 122 strikeouts against just 32 walks in 130.2 innings pitched.
Escobar, who throws in the low-90's with a solid curve, could be ticketed for High-A San Jose this season. The San Jose rotation will also likely include top pitching prospects Chris Stratton, Kyle Crick and Clayton Blackburn.
Heath Hembree and the manager's son, Brett Bochy, are the two relievers in camp who combine a high upside with a close proximity to the major leagues. Hembree has better pure stuff than Bochy, but injuries and control problems set him back last year.
Hembree dials his fastball up in the mid-to-high 90's and mixes in a swing-and-miss slider. He's struck out 143 hitters in 107.1 minor league innings thus far. However, he struggled at Triple-A Fresno last year, posting a 4.74 ERA while walking 20 in just 38 innings of work due to an elbow strain.
The skipper's kid was shut down due to arm fatigue at the end of last season, but not before dominating Double-A hitters. Bochy saved 14 games while putting up a 2.53 ERA and striking out 69 hitters in 53.1 innings of work.
Like Heston, Bochy has been able to miss minor league bats without elite fastball velocity, but in addition to his arm fatigue last season, Bochy has also undergone Tommy John surgery.
His lack of pure stuff and prior injury history could prevent him from translating his minor league success into the majors. However, the last Giants' relief prospect who came to the big leagues with concerns about his fragility and without a dominant fastball was Sergio Romo, and that turned out just fine.
Heston, Kickham and Bochy were teammates at Double-A Richmond last year, and Hembree pitched at Triple-A Fresno. All four are likely to be sent to Fresno out of spring training and potentially make their professional debut in the event of an injury or performance issues on the Giants' pitching staff.
Escobar might have the most upside of the group given his youth and stuff. However, he's got a long way to go before reaching the big leagues.
The Giants' highest upside arms—Stratton, Crick and Blackburn—will probably be in camp next year. The fact that there are several other exciting arms in camp this season speaks volumes to how loaded the Giants' farm system is in regards to pitching.
(All statistics in this article are from Baseball-Reference.com).