Baseball is a game of narrow, marginal advantages. The difference in performance between an All-Star paid a five-figure salary and a bench player paid a three-figure one can work out to a difference in reaction time measured in thousandths of a second, the sort of difference that allows one player to knock curveballs to the opposite field while the other swingsa right over them. Expressed differently, the difference can be as little as two-tenths of a run per game.

Carlos Beltran isn’t a better player than Endy Chavez because he gives the Mets a statistically notably better chance of winning any particular game, but because over 150 games the relatively small difference between them accrues and becomes enormous, worth something like nine wins and tens of millions of dollars to the team’s owners.

It’s because teams increasingly realize this that we see an increasing reluctance to trade away packages of hugely valuable young talent for two-month rentals at the trade deadline. Washington Nationals general manager Jim Bowden doubtless had some nice offers on the table for left fielder Alfonso Soriano, but deals like the one the Mariners made in 1998 — when they acquired Freddy Garcia, Carlos Guillen, and John Halama for two months of Randy Johnson’s services — just aren’t going to happen anymore. Everyone realizes that unless they get a player who pitches like Johnson did down the stretch that year, and he’s replacing an outright scrub, the difference just isn’t going to be all that big.

This brings us to the Boston Red Sox and their captain and star catcher, Jason Varitek, who injured his knee Saturday in a home plate collision, injured it again Monday rounding the bases, and should be out a month following surgery. A half-game out of first place heading into their game last night, the Sox are in the tightest race imaginable with the Yankees, who, with their recent acquisitions, look poised to pull away.

So now the Sox appear set to pick up Orioles designated hitter Javy Lopez in a trade, according to an ESPN report last night.The defensively challenged Lopez has of course spent his career as a catcher, and has in fact put up borderline Hall of Fame numbers as one. Does this trade make a difference?

“Heart and soul of the Red Sox” type arguments aside, Varitek hasn’t been all that productive this year — batting .247 BA/.334 OBA/.417 SLG, he’s been a positive contributor to the Sox offense, but hardly a crucial one. Lopez hasn’t been much better, having hit a reasonably similar .268/.316/.412, but he’s only caught 21 games.You could take that as a bad thing (if he’s hit only that well with minimal defensive responsibilities, how will he hold up under the strain of catching?) or as a good thing (he should be fresher than Varitek would have been over the next month).

Either way, it’s pretty much just a wash on offense, which makes it a good thing for the Sox. Moreover, Lopez may not be much behind the plate, but Varitek isn’t what he once was either. I hardly dismiss pitch calling and the like, especially with the Sox relying on rookie pitchers like Jonathan Papelbon and Jon Lester, but this should be a manageable problem.

The key, here, though, is that Lopez isn’t replacing Varitek, he’s replacing Doug Mirabelli, who’s on the roster only because he can catch Tim Wakefield’s knuckleball. Usually a very good hitter for a backup, Mirabelli has been just awful this year, batting .189/.255/.356 for the Sox.He’d likely be better than that if given regular duty for the next month, but you can’t blame the Sox for not wanting to find out.

On a prorated basis, the difference in offense between the way Mirabelli has hit this year and the way Lopez has hit isn’t enormous — it’s around fourtenths of a run per game — but over the course of a month it is pretty big, somewhere in the ballpark of 10 runs. As a rule of thumb, 10 runs equate to a win.

Now for all anyone knows, Lopez will go on a David Ortiz-like clutch tear and win three games single-handedly. But for all anyone knows, Mirabelli would have done the same had he been handed the starting job; the point is not that baseball is akin to accounting and that the Sox will be able to point to one game and say, “Lopez won that for us. “The point is that those are the odds as they stand now, and the Sox are doing what they can to weight things in their favor. It’s a tight pennant race, and a difference of 10 runs, especially with the Yankees having added a low multiple of that number with their recent trades, is a big deal.

The other point is that as valuable as a truly great player is, that’s how not valuable one like Mirabelli, who’s performing at a truly horrific level, is. There isn’t a lot of value in Javy Lopez right now — but there is a lot of value in not being Doug Mirabelli, enough so that Lopez could be the difference between October baseball and golf for the Red Sox. The difference is small, and maybe enough to add up to gaudy jewelry and a big parade in Boston.

tmarchman@nysun.com