The ceiling on Henry Sosa is one that has declined sharply in a short period of time. Once considered a sleeper prospect in 2007 when he made the All-Star Futures game, Sosa has had a hard time finding consistent success in his journey through the Giants system. While he may still have a future as a middle-reliever of some kind (and that isn't necessarily a bad thing to have), it's safe to say that Sosa probably won't live up to the hype he garnered back in 2006 and 2007, when he burst onto the scene in the Arizona Rookie League, SALLY and California League as a 20 and 21-year-old.
When Sosa broke into the minor leagues at 20 years old, Sosa was known for a lively fastball and for striking guys out in bunches (his K/9 was 11.4 his rookie year in the AZL). When he advanced to the SALLY and California League in 2007, he had some issues with his control, as he posted high walk numbers (61 walks in a 125.2 IP between Augusta and San Jose). That being said, he still maintained good strikeout numbers (11.0 K/9 in 63.2 IP in San Jose; 8.9 K/9 in 62.0 IP), so there wasn't too much panic about Sosa's control, especially considering he was only 20 years old
2008 was supposed to be a big transition year for Sosa (especially considering the hype he garnered by making the Futures game). Here is what Obsessive Giants Compulsive said about Sosa before Spring Training that season:
"...Our rotation could get very crowded with top of the rotation throwers in a couple of years. We have Zito, Cain, Lincecum, Lowry, and Correia currently plus Sanchez and Misch in the wings, but Sosa, Bumgarner, and Alderson could be ready in the 2010-2011 timeframe to come up and start in the majors, though I would say that there is an outside chance that both Sosa and Alderson could make the majors in 2009 - as Sabean said Alderson could on draft day - if the Giants advance them aggressively, much like they did with Sanchez, bringing them up to relieve initially..."
Unfortunately, Sosa didn't transition to the Majors as fast as Giants fans would have hoped as injury issues kept him in the lower minors for another season (he had surgery to repair a torn tendon in his left knee). After pitching 125.2 IP in 2007, Sosa only pitched 57.2 IP in 2008 and didn't show much improvement in his second go-around in the California League. His ERA was 4.31 and he allowed 62 hits and 18 walks in 56.1 IP in San Jose. While he did strike out 58 batters, his 9.3 K/9 was a 1.7 point decline from the previous season.
The health problems and regression from 2007 to 2008 probably should have been a red flag and a sign of things to come. However, the real warning signs came during his 2009 season in Double-A Connecticut. Despite posting good "traditional" numbers to start out the year (6-0, 2.36 ERA), Sosa was shut down after 14 appearances and 72.1 IP due to "arm fatigue" issues. While the arm issues were an obvious scare, what was even more concerning was his 5.5 percent strikeout rate in the Eastern League, an almost four percent decrease from 2008 and a five and a half point decrease from 2007.
Sosa bounced around between being a starter and reliever last season in Fresno, but eventually found himself regulated more to the bullpen over the course of the year. Despite his role-change, the numbers didn't change much in the Pacific Coast League: his K/9 rate only improved to 6.5 and his K/BB ratio was 1.51, the lowest ratio of his minor league career (until this year where it is 1.19, but it is only the start of the season so I won't rush to judgment too quickly).
In many ways, Sosa's career path almost resembles that of Merkin Valdez. Granted, Valdez was a higher-rated prospect than Sosa. Valdez ranked No. 40 and No. 58 according to Baseball America in 2004 and 2005. Sosa never cracked any of Baseball America's Top-100 lists. (Though in retrospect, the Giants system was so shallow that Valdez was really just the best of a meager bunch, and hence, his ranking was inflated in response. The Giants system was a lot better when Sosa arrived, and hence, he went under the radar with Madison Bumgarner and Tim Alderson commanding all the attention.)
If you look past the "rankings" though, Sosa and Valdez have had very close career paths. Valdez broke in as a raw pitcher with serious gas, but injury issues (he missed the entire 2007 season due to Tommy John surgery) and never developing as a "pitcher" (e.g. just being a "thrower" who relies on his fastball) prevented him from capitalizing on the early hype he garnered in the minors. The same could almost be said of Sosa. He had a great arm, but he's been plagued by injuries, and his command has been questionable since his knee surgery.
If you don't buy the Valdez-Sosa comparison, check out their Minor League numbers (Sosa and Valdez) and you'll see that they have had eerily similar statistics (lower K/BB ratios and lower K/9 numbers the higher they climbed in the minors). Does that mean Sosa is the second coming of Valdez? Not exactly, but I don't see Sosa faring much better than Valdez considering where he's at and how he's pitched lately, especially this year with the Grizzlies (8.59 ERA, 2.52 WHIP in 14.2 IP).
Of course, Valdez did pitch 68.1 IP in the Majors over four seasons and 69 games. If the Giants can get that out of Sosa, he may still prove to be valuable to this Giants organization. Bullpen depth is never a bad thing to have (especially considering how inconsistent middle relievers can be), and it's always nice to bolster that depth as cheaply as possible and within the system (Sosa satisfies both factors).