If there is one completely unobjectionable baseball cliche, it’s that you’re only as good as the next day’s starting pitcher.

Given the essential truth of this old saw, Yankee fans should be devastated by the news that Chien-Ming Wang, who leads the team’s rotation with a 3.89 ERA and was scheduled to start tonight against the Red Sox, will be going on the 15-day disabled list with a sore shoulder. The Yankees’ ambitions of narrowing the gap in the American League East with a convincing victory in this weekend’s series at Fenway Park should now become a great deal more modest, despite last night’s 8-6 win. Not falling further behind in the division may not be the mightiest of aims, but with Wang on the DL and upcoming series against the Angels, Rangers, and Twins, it might also be the best the team can hope for.

The fact that an injury to a rookie pitcher could end up costing the Bronx Bombers a playoff spot is perfectly indicative of the kind of season the $200 million team has had. Also telling is the specter of Mike Mussina and Randy Johnson, two of the best pitchers of their generation, alternating starts with Tim Redding and Tanyon Sturtze, who as starters rank among the worst in the major leagues. It’s true that the Yankees are only in this spot because of injuries to Jaret Wright, Kevin Brown, and Carl Pavano, but no one should have needed a crystal ball to foresee health woes for those three. In fact, given Johnson’s various maladies and Pavano’s spotty history, the team has probably been a bit lucky not to have seen more starters go down.

For a Yankee fan, there probably isn’t a more nightmarish scenario than Redding and Sturtze taking the hill against the Red Sox. But it looks as if that could happen this weekend. Redding has been penciled in to start tonight’s game in stead of Wang, and though manager Joe Torre has yet to decide whom he will pitch on Sunday, Sturtze seems the most logical choice. Other possibilities include Darrell May, author of a 1-4 record and 6.22 ERA this season with San Diego and New York; Sean Henn, 0-3 with an 11.12 ERA in three big-league starts, and “somebody from the minor leagues,” as Torre put it yesterday.

Sturtze has been surprisingly good as a reliever this year, but his final inning in his sole start, against the Orioles, is entirely typical of what can be expected. Looking as if he’d lost all ability to throw the ball over the plate, Sturtze hit a batter, walked three, and gave up a single while only getting two outs. There’s nothing dishonorable about giving up four runs in 4 2/3 innings against Baltimore’s excellent offense, but Sturtze’s refusal or inability even to give himself a chance at success by challenging hitters leads to the most maddening sort of failure of all, where a pitcher beats himself rather than being beat by hitters.

Sturtze, though, may as well be Roger Clemens by comparison to May and Redding, who came over from the Padres in the Paul Quantrill trade. Redding was once a top prospect with the Astros and is now essentially a batting practice pitcher. His 9.10 ERA and 0-5 record, somehow compiled in just 29 2/3 innings, dramatically understate how bad he’s been this year. Those numbers were run up while he was pitching for the Padres, who play in perhaps the most pitcher-friendly park in baseball.

In 15 1/3 innings at San Diego’s Petco Park, Redding pitched respectably, putting up a 4.11 ERA and giving up one home run. His 14 1/3 road innings give a truer picture of his abilities – he gave up five home runs, and walked nine against nine strikeouts.

Redding has a few problems, but the main one is that he’s a sinkerballer whose sinkerball doesn’t sink. Hitters either take it for a ball – in which case he starts offering up curves and sliders that have nice breaks but which he can’t really command – or they whack it out of the park. He’s the sort of pitcher one can imagine the Braves’ Leo Mazzone fixing, but unfortunately for Redding and the Yankees, pitching coach Mel Stottlemyre is no Mazzone.

To win this weekend, the Yankees will have to hammer the Red Sox’ pitching. They may well be able to do that – the way the rotations line up right now, it looks like Redding and Sturtze will be matching up against David Wells and Wade Miller, both of whom sport ERAs over 5 this season. But they’re also going to need Johnson to go deep into Saturday’s game, because Sturtze’s foray into the rotation could cause a chain reaction in the bullpen, removing the main middle-inning setup man and potentially leaving close games to the likes of Scott Proctor.

Like so many of the Yankees’ problems this season, their tattered rotation is partly the result of bad luck, but mostly the result of bad planning. Any team whose sixth starter is a key setup man is asking for a load of trouble, and the Yankees are getting it.

It’s one thing to lack an attention to detail in matters like having a decent pinch-runner or defensive backups for the middle infield, but actually constructing your roster in such a way that there isn’t anyone trustworthy to pitch the middle innings in a tight one in the biggest series of the year is indicative of something a bit worse than absent-mindedness, something a lot more like arrogance.