For starters, Jason Bay is a horrendous defensive outfielder. According to Fangraphs:
Jason Bay leads off this series with his 64.9 runs below average. His defense is so bad in such a non-premium defensive position that he was merely a replacement level player in 2007 when his bat disappeared.His fielding ratings over the last three years were -11.5, -18.4, and -13.0. You can believe in the numbers or not, but for the sake of comparison? Manny Ramirez had -18.6 and -21 his last two full seasons with the Red Sox. Carlos Lee, a big man with a bad defensive reputation, has only posted rates of -4.4 and -5.4 the last two seasons. Bay is way worse.
I'm not a professional scout, but Bay just looks unathletic. Jason Bay is already 30 years old, though he seems younger, because he has not been in the league for long. It is unlikely that he will age gracefully.
He's a terrific hitter, but he's not without his flaws. He doesn't hit for a high batting average. Players with good isolated power and good plate discipline, but who have poor batting averages, tend to fall off more abruptly later in their careers. And he NEEDS to stay a great hitter in order to be worth a big contract because his defense is so bad.
He's already had an injury-marred season where his production fell off a cliff -- in 2007, when he batted only .247 and had an OPS of 746. At the age of 28, people wondered if he was done.
That's not the kind of player I'd like to see the Mets give a long term contract to. How will the Mets look in 2011 if we signed Bay at that price? He'll be a bad defensive left fielder, who is 34 years old, making $18 million per year at least. He'll be the third most expensive Met under contract, after Santana and K-Rod.
Is this the guy we want to occupy 10% of our payroll with? Is this the kind of player we want to shape the rest of our roster around? Is he a cornerstone for the next Mets World Series team? I don't think so.