Remember when Jason Bay was good? It was totally a thing. Between 2004 and 2009, the guy hit .280/.375/.519. And that's including a season in which he hit .247/.327/.418. He averaged 30 home runs a season, got on base, and even flashed some speed every now and then. Awful defense notwithstanding, he was one of the best players in baseball for a couple years there. No joke.
And then the Mets came. They were all, "This Jason Bay kid seems pretty good at baseball. If we sign him, I bet he'll continue to be good at baseball." So they signed him. And, because these kinds of things happen to the New York Mets, Jason Bay stopped being good at baseball (relatively, of course). He was riddled with injuries, his power fell off the face of the earth, and his house was constantly egged and TP'd by Mets fans (unconfirmed but likely).
The Mets got tired of Jason Bay being a not-good player, and they told him to leave. So he left. Then the Seattle Mariners were all, "Jason Bay is bad, but he used to be good - maybe he'll be less bad if we sign him." So they signed him. And that brings us to now. Bay went from signing a four-year, $66 million contract in 2009 to a 6-figure contract in 2012. It couldn't get much worse for Jason Bay (again, relatively).
But could it get better? Let's find out.
The primary reason for Jason Bay's decline was a huge drop-off in power. Consider the following graph, mapping his wRC+ to his ISO:
Though other factors surely played a part in Bay's downfall, his sudden inability to hit baseballs a long way seemed to be the primary determinant.
Two immediate explanations for this power outage come to mind:
1. Citi Field
We can dismiss the first pretty quickly. Citi Field depressed home runs hit by right handed batters by about 8% from 2010-2012, but Bay's HR/PA dropped over 50% between '04-'09 (4.8%) and '10-12 (2.3%). In fact, all but about four of Bay's 36 home runs in Fenway in 2009 would have also been home runs at Citi Field, as illustrated below:
So it wasn't Citi Field. In that case, the obvious alternative is injury. Bay had two types of major injuries with the Mets: concussions and rib injuries. He missed about 60 games in 2010 from a concussion, the first few weeks of 2011 due to "ribcage discomfort", about a month and a half in 2012 with a fractured rib, and another month last season from yet another concussion.
This would seem to explain a lot, except for one small detail: Bay's ISO dropped before these injuries occurred. His 2010 concussion ended his season at the end of July, but he had played 95 games, and accrued over 400 plate appearances, up to that point. In those plate appearances, he had a home run rate of 1.5%, HR/FB of 5.1%, and an ISO of .144 (down from .269 the previous year).
As Fangraphs' handy sample size reference tells us, 400 PAs is enough to make home run rate and HR/FB (but not ISO) reliable. So Bay's depressed numbers in 2010 didn't stem from Citi Field or injury (as far as we know), and are a large enough sample to indicate a real drop in power.
Still, something must have changed to explain such a drastic decline in effectiveness from 2010 on. Let's compare Bay's plate discipline numbers from '08-'09 to his numbers from '10-'11.
You can spot the big differences pretty easily. Bay swung at way more pitches outside the zone, and made more contact on the ones that he swung at, in 2010 and 2011 than in 2008 and 2009*. The consequences of swinging at pitches outside the zone are obvious: a combination of fewer walks, more strikeouts, and weaker contact. While Bay did see a drop in walk rate in '10 and '11, his strikeout rate actually improved. This isn't really a positive, however, as it is most likely tied to the second change: O-Contact%.
*Keep in mind that although his O-Contact% was about the same in '08 and '10, he swung at more pitches outside the zone in '10, increasing the impact of the elevated O-Contact%.
Though swinging at more pitches outside the zone seems, intuitively, the more likely of the two to influence offensive production, it's not. There is very little correlation between O-Swing% and ISO or wRC+; however, O-Contact% has a stronger relationship with power. Consider the following plot of ISO against O-Contact% for all qualified seasons since 2002:
As you can see, we find a clear negative relationship between the two metrics. Generally, the more contact that hitters make on pitches outside of the zone, the worse their power will be. This makes sense intuitively, as batters that swing harder will likely whiff on more pitches, but will also hit the ball farther when they do make contact. Similarly, contact on pitches outside the zone is likely to be weaker than contact on pitches inside the zone.
This is a step towards explaining Jason Bay's miserable failure with the Mets. For whatever reason, Bay began to swing at more pitches outside the zone, and make contact with more of those pitches, even before he was plagued with injuries. 2011 was even worse, as Bay's O-Contact% jumped to 66.6% while his O-Swing% remained at 27%.
Now to be clear, these numbers aren't poor relative to the rest of the league - in fact, they're above average. However, they indicate a shift in plate discipline for Bay. Maybe he was trying too hard. Maybe he was trying to make more contact to fit his new environment - I'm not quite sure. But whatever changed clearly showed in his performance.
There's good news for Bay, however. Take a look at those same plate discipline numbers, now with 2012 included:
It's not ideal, but in 2012 we saw some improvement from Bay, at least as far as O-Swing% and O-Contact% are concerned. Of course, he stopped swinging at pitches inside the zone as well, but this may still be a sign of good things to come for Bay. The improved O-Swing% indicates more patience and a better eye, and the improved O-Contact% indicates better contact.
There's a solid chance that all of this is hogwash, that Jason Bay is never going to be even close to an average hitter again. However, the issues that he had in the first two years with the Mets as far as plate discipline is concerned seem to have improved in 2012. Maybe the Mariners noticed this as well, and are taking a shot that it's a sign of good things to come. No one should expect another 130 wRC+ campaign anytime soon, but maybe, just maybe, Bay can be an above average hitter once again.