Hernandez was a third-round pick of the Mets in 2004. He was traded to the Marlins by the Mets to acquire Paul Lo Duca. He dominated the lower levels as he moved his way up, but seems to have hit a little bit of a wall and needs to work his way through it. He just turned 22, so there's plenty of time for him.
As for Hernandez' tools, he throws his fastball consistently in the low 90s and features a decent breaking ball that is devastating at times. He also has a nice changeup. Hernandez threw 120 innings in A+ ball in 2006 and followed it with 153+ innings in AA last season. He already has nearly 550 innings under his belt in the minors, so he's showing himself to be pretty durable on his way up (derailed only slightly by this year's intercostal strain).
Hopefully, this year's numbers in Albuquerque are an aberration caused by his injury and the thin New Mexico air (they play at a higher elevation than Denver). Gaby has a good shot of becoming the second Hernandez in the Mariner rotation in the future --- a pretty nice snag in exchange for a 38-year-old with no future with the organization.
2006: 120 IP, 120 H, 35 BB, 115 K, 7 HR, 3.67 ERA, 1.29 WHIP with A+ Jupiter
2007: 153.2 IP, 144 H, 56 BB, 113 K, 14 HR, 4.22 ERA, 1.30 WHIP with AA Carolina
2008: 64.2 IP, 94 H, 26 BB, 54 K, 14 HR, 7.24 ERA, 1.86 WHIP with AAA Albuquerque
2008: 23 IP, 21 H, 4 BB, 17 K, 3 HR, 4.30 ERA, 1.09 WHIP with AA Carolina