The Boston Red Sox might very well have a difficult time winning games during its present series against the Seattle Mariners: While the Red Sox are 21-5 at home, they’re only 10-17 on the road. But — despite having one of the best young power pitchers in baseball, Felix Hernandez, and home field advantage, which is particularly strong during this year’s regular season — the M’s have a poor offense. King Felix aside, they also have the worst pitching-defense combination in baseball. As the Yankees discovered over the weekend, everyone looks good against the Mariners.

Seattle’s mediocrity — which team general manager Bill Bavasi recently insisted in a Condoleezza-like way that no one could have predicted, while many actually did — constitutes a dangerous illusion for the supposedly improving Yankees. Their season-best five-game winning streak, halted yesterday by the Baltimore Orioles, was achieved through an impressive three-game sweep of the M’s at Yankee Stadium, in which the recovering Bombers outscored the visitors 31-13. The Yankees also took two prior games from the fading Orioles who, after yesterday’s win, have a 10-13 record on the month, despite playing seven games against the Royals and Nationals (games against the Angels, A’s, Red Sox, Yankees, and Rays exposed the inability of a thin offense to support a generally decent pitching staff).

The question that needs to be asked, then, is: Did anything really change over the weekend?

Over the last 10 years, it has taken an average of 99 wins to claim the American League East title, while the league’s wild card has required an average of 96 wins. Hard as it is to believe, the Rays have the league’s best record at 30-20, a pace for 97 wins, while the slightly slumping Red Sox lead the wild card race at 31-22, a 95-win pace. If these two clubs don’t raise their level of play, then teams like the Yankees, who are looking to come from behind and grab a postseason share, will receive a slight discount on their aspirations.

Yet the chances of the Yankees achieving the October dream haven’t appreciably strengthened, despite the team’s little winning streak. Getting one game ahead of the current division-leading pace would require them to go 73-38 over the rest of the season, a pace equivalent to 107 wins over the course of a full season. Topping the wild card takes only a slightly more forgiving 71-40 finish, tantamount to a 104-win pace over a full season.

Joe Girardi’s team hasn’t shown anything akin to the ability that could sustain that kind of pace. One small reason is their vulnerability to left-handed pitchers. Going into yesterday’s game, the Yankees were hitting just .245 AVG/.318 OBA/.354 SLG against left-handers, and three hits in seven innings against Garrett Olson didn’t help those numbers any. The return of Jorge Posada to the lineup, presumably in a couple of weeks, should help this a bit, as Posada is a career .301/.383/.500 hitter against southpaws. But he won’t solve the problem alone, as the Yankees have a number of players who need to step up to southpaws, including Jason Giambi, Bobby Abreu, and, emphatically, Melky Cabrera, a supposed switch-hitter who is a career .254/.324/.340 hitter against port-siders — but is batting only .190/.262/.328 against them this season. Shelley Duncan and Morgan Ensberg were supposed to be part of the solution here as well, but neither has performed to date, one reason why the Yankees are just 8-10 when the opposing starter is left-handed.

This problem is easily fixed, at least in theory: The game produces right-handed platoon hitters in abundance, and if the Yankees tire of trying to make do with their current options, they shouldn’t have much trouble finding others. But the problems with the club remain inherent in its construction. Joba Chamberlain is racing to join the starting rotation, but it’s not certain that the rest of that group will show the consistency necessary for him to make a crucial difference. Mike Mussina and Andy Pettitte have been streaky; Chien-Ming Wang has been hammered in his last two starts (by two of the worst offenses in the league), and Ian Kennedy pitched “better” in his last appearance. But his four walks in six innings argues that his Steve Blass-like control problems remain an issue.

The defense remains poor. As Giambi and Robinson Cano have heated up, Johnny Damon (.232 with two walks in the 14 games since his last home run) and Cabrera have vanished before our eyes. Despite Jose Molina’s game-winning hit on Sunday, the catcher substitutes have been murderously poor, and the Yankees will be stuck with them for a minimum of two more weeks, as Posada works himself back into shape. Most troublingly, the day when Derek Jeter’s offense no longer pays the cost of his defense seems finally to have arrived.

The Yankees may show some life again in June, with series against the Royals, Padres, Reds, Pirates, and Mets. But victories in those games, as with the sweep of the Mariners, mean only that the Yankees are not a bad team. They will not, however, elevate the team to contender status. The operative word in the Bronx should still be “rebuild.”

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