There's been a lot of coverage of the Manny Ramirez - Jason Bay trade. I thought it'd be good to look more closely at his stats, to divine just what we can expect from him, and how good or bad the trade is.
Offense
Offensively, Jason Bay is NOT Manny Ramirez. In his best season, 2005, Bay hit .306 / .402 / .559 (150 OPS+) with 35 homers. In 2000, Manny hit an earth-shattering .351/ .457 / .697 (186 OPS+), having not only his best year, but one of the best all time by any slugger.
Ramirez Career Hitting: .312 / .409 / .590 (154 OPS+)
Bay Career Hitting: .281 / .375 / .515 (130 OPS+)
So Jason Bay in his prime is a good hitter, not an otherworldly one like Manny was. The real question is how Bay will compare to Manny going forward. Age is a factor here: Bay is 29, right in the peak years for most players (28-32), while Manny is 36. This year, their performances have been remarkably similar.
Ramirez 2008: .299 / .398 / .529 (140 OPS+), 20 HR
Bay 2008: .282 / .375 / .519 (135 OPS+) 22 HR
As poster Fugimaster24 pointed out, the Pirates have a strong lineup (NL's 3rd in runs scored), so it's unlikely his numbers will shoot up just because he has 'protection.' Bay may enjoy leaving the spacious confines of PNC Park - his career slugging is 23 points better on the road (although this season it's way better at home, .571 to .455).
One concern I have is Bay's performance against lefty pitching. Over his career, the lefty-righty split isn't significant (18 points of OPS), but this season he's really struggled against southpaws. His 2008 OPS against them is .679 (vs .953 against righties) and he's hit only 2 of his 22 homers off lefties. The culprit may be bad luck, with balls in play not dropping in for hits (.226 BABIP). Manny's also been scuffling against lefties, but not to the same extent (.737 OPS).
Another worry is that Bay may not hit well in Boston, or against the American League. His career numbers in Fenway are way too small to count (3 games), and he hasn't played many games against the AL overall. In 64 inter-league games, he has a .762 OPS.
One area where we can expect immediate improvement is on the basepaths. Bay is much faster than Manny, who was never a speedster (35 SB in 66 lifetime attempts). Bay had a career year on the basepaths in 2005, swiping 21 bags while only being caught once. He was 11 for 13 in 2006, but only 4 for 5 in 2007, the year he battled knee injuries. So far this season he's 7 for 7 in stealing opportunities. Bay's improved speed should also save some runs on the basepaths; Manny was a bad baserunner, often dogging it to first or being overly aggressive in heading to second or home.
Fielding
Manny was atrocious by almost any metric, but his replacement has also struggled defensively. By fielding win shares, Manny has been better the past two seasons (1.7 and 2.7 to Bay's 1.0 and 1.9), while Bay was better in 2006 and 2005 (3.6 and 3.3 to 2.1 and 2.9). A strength for Bay was getting outs on balls out of his zone, which is measured by the stat OOZ. Over the past three years, Bay's OOZ has been 56, 47, and 38; in the same span, Manny's OOZ has been 38, 35, and 14. Also, Bay's revised zone rating (RZR) has been consistently superior to Manny's; Bay's average RZR the last three years has been .857, to Manny's .714.
It seems to me that Bay has better range than Manny, and could prove to be a better defender. That said, Bay is no Ellsbury in left, and he may end up hurting us on the field, especially adjusting to the Monster.
Agent & Conclusions
Jason Bay's agent is Joe Urbon, not Scott Boras. The specter of Boras has been haunting discussions of Bay's pending free agency (offseason 2009-10), and it really shouldn't. Another of Urbon's clients is Grady Sizemore, who signed a massive extension with Cleveland. It's possible that Bay will do the same if he likes Boston and Theo shows him the money.
So Jason Bay is a good slugger who plays poor defense, but with better range than Manny. I'd say this year he'll be worth between 75-90% of Manny's offensive production. Playing better defense might make him more valuable than Manny; the Hardball Times already ranks Bay as more valuable, with 19 win shares to Manny's 15.
Honestly, this was the best return we could have hoped for. Ramirez is 36 and very expensive - to turn him into a year plus of Jason Bay is pretty good. Top prospects miss all the time, and draftees often never succeed in the bigs (looking at you, Hansen). I'm excited about this deal, and optimistic that Bay will succeed in Boston; he may even become Manny's long-term replacement.
Resources:
Baseball-Reference: Bay career stats ; Manny stats
Baseball Prospectus: Bay stats ; Manny stats
Hardball Times: Bay stats ; Manny stats