If Al Leiter never pitches another game for the Yankees, if he never pitches another good game, he will have done the Yankees an inestimable service in their pursuit of the 2005 postseason. By winning on Saturday, the Yankees assured themselves of leaving Boston with no less than a draw despite Friday’s 17-1 massacre. Thanks to Leiter, the Yankees left Boston having taken three of four games at Fenway from the front-running, defending champion Red Sox.

Despite their status as reigning trophy-holders, the Sox are a flawed team, so perhaps it would be an overstatement to call New York’s showing this past weekend miraculous. But the Yankees are a flawed team, too, one for which – perhaps until last night – 60% of the starting rotation is purely hypothetical. Friday’s Tim Redding-authored Little Bighorn (with a Worst Supporting Actor nod to Darrell May) could easily have been a preview of debacles to come over the rest of the season. With the professional pitchers indefinitely indisposed, roughly every other night the Yankees will be forced to pitch someone replacement-level or worse, interspersed with the occasional found veteran washout.

It all reeks of the 1987 Yankees, with Steve Trout, Pete Filson, Brad Arnsberg filling the gaps among Tommy John, Rick Rhoden, and Ron Guidry. It’s a good recipe for a quick withdrawal from the pennant race.

Not coincidentally, that 1987 team’s .549 winning percentage is not far removed from the 2005 Yankees’ .544.

Of course, Leiter’s surprising 6.1 innings, three hits, one run, three walks, and eight strikeouts do not mean the test is over. The month of July does not get any easier for the Yankees. If anything, it gets harder. There are seven games with the Angels to contemplate, as well as three at Texas (traditionally a difficult place for the Yankees to win) and three at home against the Minnesota Twins.

The spot in which Redding pitched, now to be filled by the catch of the day, comes around again on Wednesday, and the irony is that though Kevin Brown may return to occupy that space, and though Carl Pavano may return not long after, it is not clear what the Yankees will get from these pitchers. It is almost certain that they will do a better job of keeping the Yankees in games than did Redding, May, or Sean Henn, but there is still a great distance from those pitchers – the first two are meatball artists, the latter still an apprentice – to league-average reliability or even Leiter’s unexpected dominance. That seems too much to hope for.

The Yankees have problems to solve on offense and defense simultaneously, though fewer in number than ever before thanks to Jason Giambi’s rediscovery of his vitality. The first-base and designated-hitter problem seems to have solved itself, at least with Giambi occupying one or the other at any given moment (though, as ever, Giambi has been a lesser man when DHing, batting only .234/.393/.346 when not wearing a glove, against .347/.492/.644 when playing the field).

As good as the Yankees offense has been of late, batting .302/.370/.528 in the month of July, it makes little sense for the team to continue through the remainder of the season receiving subpar production from this purely offensive position.

Center field is a more serious problem. The Melky Cabrera experiment is a failure, as it was doomed to be from the start. The 20 year-old Cabrera may someday be a fine player – certainly his advanced proficiency at so young an age argues in his favor – but the time is not now. His defensive problems are well chronicled and dovetail with scouting reports reminding us that he was not the next coming of Andruw Jones.

On offense, Cabrera has proved to be terminally impatient, swinging at nearly every pitch that comes his way. The issue is not that he doesn’t take walks – as Robinson Cano has shown, an impatient hitter can contribute if he can hit for a high enough average. Cabrera may yet show that he – like Cano, who struggled after being called up – can adjust. But Cabrera’s defensive struggles make it much harder to wait for his bat to come around.

Bernie Williams’s glove cannot be expected to be reborn, like Giambi’s, just because this is the moment that the Yankees need him the most. His bat is not what it once was, but would be sufficient if only Williams was a defensive asset. But those days are over, and now his return to regular play will ensure that in some games, perhaps important ones, his lack of range and weak arm will undo the team.

What the Yankees did in Boston this past weekend was real and should be celebrated. Still, the winning-on-a-wing and-a-prayer approach cannot pay dividends forever; more changes must come if this is to be another turning point rather than the high point of an abortive pennant run. More changes, yes, and a lot of luck.

Mr. Goldman is the author of “Forging Genius,” a biography of Casey Stengel.