IMO you can't rule out his being rushed. At age 22 in 2005 he pitched in four leagues, High A Wilmington, AA Portland, AAA Pawtucket, and then Boston. He did very well until he hit Pawtucket where he had an ERA of >5.50 in 40 games. I can't imagine why they brought him up then, but there must have been a reason.
For some reason I remember the game he imploded - against Seattle. The second game of a DH. 1/3 inning, 2H, 2BB, 3ER, 1 HR (a GS IIRC) and was essentially never heard from again in Boston. I remember thinking, "C'mon.. will you pull this guy for God's sake. He's just a kid. You can't leave him there!"
Here's something I didn't remember. Meredith was traded to the Padres for Doug Mirabelli.