I like Bay and think that if he hadn't spent his first few years in Pittsburgh, he'd be a bona fide star. He's a .282/.375/.516 player who averages around 30 HR and 100 RBI year-in-year out, and that's with the bulk of his career having been spent on putrid teams in Pittsburgh. Bay strikes out too much, but he also draws a fair share of walks as well. I think that, in some ways, Bay is a great fit for the Red Sox lineup -- he's much more of a Moneyball type player than Manny is; he gives them solid numbers in all categories while doing at about 30% of Manny's cost (Manny's a $20M/yr player; Bay's at about $6M/yr) while giving them another year at a below-market cost (7.5M in 2009).
None of that is to say, however, that Bay could ever be called even with (much less better than) Manny. Statistically alone, Manny's average season (over a much longer span of time) blows away Bay's best season -- averaged over 162 games, Manny's numbers produce .314/.411/.593 with 41 HR and 133 RBI. So, Manny's average year has him hitting about 30 points higher, getting on base substantially more often (.375 vs. .411) and racking up a boatload more total bases than Bay (.516 to .593), while hitting about 10 more homers and driving in about 30 more runs. Those numbers are insane and the difference is gigantic (particularly at that level of player). Even if you factor in the costs that Manny imposes on a team defensively or on the basepaths, he's unquestionably the more valuable player, in my mind. It's really not even close.
It's interesting to see, though, that Red Sox fans seem to have accepted that the expectations on Bay have to be lower than they were on Manny. Manny might be the greatest right-handed hitter of this generation (although ARod's regular season numbers are close) and probably is among the top 10 or 14 right-handed hitters of all-time. It's awfully difficult to replace that guy with a comparable guy unless you get guys like Hank Aaron or Willie Mays in their primes.
For fun, I went to check that claim out; based on 162 game averages (not perfect, particularly given that the retired guys' numbers incorporate their end of career decline) and neutralized numbers, Manny's pretty much the same guy offensively as Aaron and Mays:
Mays: .310/.394/.572; 37 HR, 108 RBI
Aaron:.313/.382/.568; 38 HR, 121 RBI
Manny: .303/.398/.571, 38 HR, 121 RBI
I couldn't reasonably argue that the drop from Manny to Bay is a small one, even considering every aspect of the game.