Pedro Hernandez was traded from the Padres to the White Sox in the Carlos Quentin trade. The White Sox then traded him in the middle of the year in part of a deal to get Francisco Liriano (who gave the White Sox .7 fWAR, a lot of strikeouts, but too many walks with too many homers).
Hernandez is 23 years old and is a short but stout lefty (5-10, 200). He is fastball heavy (upper 80s to low 90s, sits about 90 MPH), but it doesn't seem to play as well against righties, especially when he tries to keep it away. He likes to throw the fastball high, but moves it inside and outside. It looks like he has decent looking command with it. Hernandez also throws what looks like a sinker that he keeps low with some okay movement.
His main off-speed pitch is a change that he can throw for strikes. He will throw it to lefties as well, and tries to locate it on the outside corner to them, with pretty good success. It doesn't have great movement, but the location, speed differential, and ability to sell it with his arm action makes it a pretty good pitch.
He also mixes in a slider that dives into the dirt pretty sharply. I liked the break on it, but he really didn't throw it much when I watched him.
Statistically, Hernandez doesn't have big splits, which surprises me. I guess he locates the change well enough to keep hitters off balance. However, he does not have a good ground-ball rate, despite good looking command. That is at least somewhat concerning and it is not as if he makes up for it with a lot of infield fly-balls.
Overall, he throws a lot of strikes (over 66%) and at-bats don't usually last very long (3.48 Pit/PA over the last two years). He just doesn't walk batters (walk percentage under 5% over the last two years), but he doesn't strikeout a lot of people either.
Hernandez seems to have the mix of pitches and command to be a starter in the big leagues. It is very doubtful that he can be average, but he can certainly be acceptable. The question will obviously be whether or not he is too hittable. He is the type of pitcher who succeeds in AAA only to struggle in the Majors, a pitchability lefty with mediocre stuff. I think I am bullish on Hernandez though, I could see him being a capable above replacement starter in the Majors. He seems to be the kind of guy who would benefit from a pitcher friendly park more than an average pitcher. According to @Toritap's park factors, Target Field played slightly in favor of pitchers this year.