It may be a long season, but a week can still be a long time.

A week ago the Yankees were reeling under the weight of a 1–8 stretch, catastrophic pitcher injuries, and threats of a managerial firing. Had you told a fan last Monday that his team was about to see top prospect Phil Hughes go down to injury, that fan would have asked for directions to the nearest tall building or bridge. As is, two series against the fairly unimpressive Rangers and Mariners had the fairly predictable effect of making the Yankees look about as good as they are. They won taut, close games, overcame bullpen misadventures, pounded the ball wherever it was pitched, and ended up taking five of six games. Happily enough, all of this served as mere prelude to yesterday’s crowdpleasing announcement that the Yankees have even managed to fill Carl Pavano’s spot in the rotation.

Leaving aside the absurd, joyous theater of the greatest pitcher of all time secretly arriving in Yankees Stadium to publicly announce a secret deal he’d secretly struck with Yankees brass during the weekend — all of which was kept highly secret, mind you — we’re left with a perhaps only marginally relevant but still intriguing question: Just how much of a difference is a 44-year-old Roger Clemens going to make, anyway?

The short and unsurprising answer is, “A lot.” It seems implausible that any on-field performance could live up to the hype resulting from the unexpected marriage of the game’s most mythic figure and its most self-aggrandizing team, but as shrewd a bit of marketing as this is, it’s a lot more impressive as a baseball move.

Clemens, who agreed to a $28 million, one-year deal yesterday, is still just about the best starting pitcher in baseball, at least per start. During the last three years, he’s pitched 539 innings with a 2.40 ERA; by comparison, Minnesota’s Johan Santana, universally acknowledged as the best starter in the game, has pitched 693 innings with a 2.93 ERA.

That innings difference is, of course, massively important. Clemens is — as one would expect of someone who would have been a first-ballot Hall of Famer had he retired a decade ago — a six-inning pitcher, and his body can’t handle the grind of pitching from spring training through October. Still, the various concessions and accommodations he and the Houston Astros made because of his age had the unexpected effect of allowing him to pitch at an unparalleled level when he was able to take the mound. Assuming that the Yankees concede and accommodate appropriately, there’s no reason to expect much less of him over the last four months of the season. Tack a run onto his ERA to account for the switch from the less difficult league in which pitchers hit to the league in which David Ortiz and Travis Hafner hit, and you still have a pitcher capable of allowing 3.40 runs per nine — exactly what Santana has done so far this year.

Clemens’s innings will be coming at the expense of no. 5-type pitchers like Chase Wright, the hapless prospect who gave up four straight home runs to the Red Sox two weeks ago, and the garbage time relievers who mop up after Wrighttype pitchers get knocked out in the fourth inning. No. 5 pitchers are pretty bad — an analysis of how they actually performed last year by Jeff Sackmann of www.hardballtimes.com showed that their average ERA was 6.24. Assume that the pitchers who would otherwise have taken up the 120 or so innings Clemens will pitch would have done about this well, and the difference comes out to about 40 runs, or four wins, during the last four months. That’s an enormous difference, comparable to having opened the season with Roy Halladay in the rotation instead of Mike Mussina. Clemens’s impact on the pennant race, to put it another way, should give the Yankees an advantage over the Red Sox roughly comparable to the Sox’s lead in the standings. And none of this accounts for the impact he’ll have on the bullpen. Last year, Clemens pitched at least five innings in every one of his starts. A horse like that makes it a lot easier to schedule days off for overworked relievers, which is one of the Yankees’ main problems these days.

In sum, and disappointingly for the cynics among us, all the operatic drama we’ll endure over the next month as Clemens readies himself to take the mound will actually be proportional to how important his arrival in the Bronx will be.

Wonderful as all this news is for Yankees fans, there’s a downside as well, and it has nothing to do with Clemens the ballplayer. As some might recall, at the very end of last season the Los Angeles Times reported that Jason Grimsley gave an affidavit to federal investigators in which he said that his source for performance enhancing drugs was Clemens’s (and Andy Pettitte’s) trainer. Clemens and Pettitte responded with odd nondenial denials. Clemens, for instance, turned back questions about human growth hormone by noting that he’d never failed a drug test. There is no test for hGH.

Senator Mitchell’s inquiry into drug use in baseball is going to make its report later this year. The feds have flipped a former Mets clubhouse boy turned drug peddler, who apparently named 85 names currently playing in the majors as clients. Barry Bonds is going to break Hank Aaron’s career home runs record sooner than anyone thought. Drugs are going to be a bigger story this summer than they ever have been before.

With all that, it’s an open question whether the biggest story around Clemens is going to be his dramatic return to baseball’s biggest stage at the time of the Yankees’ greatest peril, or whether it’s going to be the rumors and accusations that have bubbled up in various investigations. I say this not in the spirit of accusation, but of caution. There will be operatic drama to spare regarding the prodigal legend in the next few months. I’m far from certain that all, or even most, of it is going to focus on what he does on the mound.

tmarchman@nysun.com