In the last week, the Yankees won a battle. The war remains very much in question.

The traditional formula for winning a pennant is that you need to stomp the bad teams, win a majority of your games against the average ones, and try to break even against the best. The two victims of the Yankees’ six-game winning streak, the Pirates and the White Sox, despite each having some strengths in the starting pitching department, fall into the category of teams that clubs with realistic aspirations of winning must beat, especially one playing as far from behind as the Yankees.

The inarguable good news is that the streak, combined with some recent choppy play on the part of the Tigers and Red Sox, has put the Yankees back in a position where the postseason isn’t just a fantasy on the part of pinstriped front office types hoping to keep their jobs for another season. While the 9.5-game gap separating New York from Boston is still unlikely to be surmounted, the Yankees remain very much alive in the wild card race, where they trail the Tigers by just 5.5 games. With 101 contests left to play, including eight with Detroit (as well as three games apiece with Oakland and Seattle, the other wild card contenders in front of them), the Yankees will have ample opportunity to bid for a spot in October’s circus.

If those games are to mean anything, the Yankees will have to beat up better teams than the two refugees from the Central divisions they clobbered last week. The Yankees were able to survive shaky or abbreviated outings from their starting pitchers because they were facing teams with very light offenses and bullpens that rank among the worst in baseball. Their hitters were unable to exploit the Yankees’ own shaky relievers, or a Roger Clemens who hasn’t yet found his sea legs. With so many Yankees currently hot at the plate, these games became contests of attrition that the Yankees could win.

The Diamondbacks and Mets, the next two teams on Bud Selig’s annual drive to dilute the integrity of the pennant races through interleague play, lack those weaknesses. The Diamondbacks will bring Brandon Webb, the reigning National League Cy Young award winner, to counter Chien-Ming Wang with some groundball magic of his own (when the two face off tonight, outfielders on each team should remember to bring a small paperback novel with them when they take the field — they aren’t going to have much to do). Lefty Doug Davis fights his control, but has upped his groundball rate and has become very stingy with the home run. Livan Hernandez, who will pitch the middle game of the series against Mike Mussina, has not pitched well on the road (4.71 ERA vs. 2.66 at home), but as always he’s durable, certainly more so than Mussina, who is on Joe Torre’s quick hook list, and he’s nearly as stingy with the home run ball as Webb and Davis.

The Diamondbacks haven’t hit much; part of their lineup won’t take a walk even at gunpoint, and they don’t have a big power bat in the middle of the order. Still, their bullpen has been solid, and if the Yankees can’t shake their starters early it’s the Diamondbacks that are in better position to win wars of attrition. Keep in mind that when Joe Torre didn’t pull Sean Henn when he loaded the bases in the fifth inning on Sunday — just about any manager, including Torre, normally would have — because he had no other options. Though Mariano Rivera seems to have righted himself and Brian Bruney has been valuable, the Yankees bullpen is like one of those greasy spoon restaurant bathrooms that you can tell hasn’t been cleaned since 1991: You don’t want to go in there unless you have absolutely no choice.

The Mets are the most rounded team the Yankees will face between the Red Sox at the beginning of this month and the Angels at the beginning of next month. The Yankees may be getting them at a good time; injuries have limited them of late, and they’re just 2–7 on the month. Weaknesses in the starting pitching department that derailed the Mets’ postseason ambitions last year may be cropping up now, but more than anything else it’s the offense that has gone cold. Only David Wright is hitting with any real élan right now. In this context, yesterday’s return of Shawn Green, a player the Mets might eagerly have disposed of in spring training, suddenly takes on real importance.

The Yankees have finally met the minimum basic requirement for competing over the long baseball season, winning with some consistency. Now they have to prove they can do it when they aren’t facing patsies. Until they do, it would be premature to conflate a victorious skirmish with a reversal of fortune.

Mr. Goldman writes the Pinstriped Bible for yesnetwork.com and is the author of “Forging Genius,” a biography of Casey Stengel.