The Boston Red Sox.
Think about it. If the Red Sox were bumping along at a pace just over .500, the holes in the Yankees' game would not be causing so much wailing and gnashing of teeth in the Big Apple. OK, there would still be the gnashing of the teeth, but the wailing would be way down.
Half of what makes up the Yankees' early-season plight is their own atypical 20-24 record. But the half that puts them in a dramatically difficult spot, a position that doesn't appear to be getting any easier, is the half supplied by the play of the Red Sox.
With their 7-3 victory on Tuesday night, the Red Sox moved to 31-14 and were once again 10 1/2 games ahead of the Yanks in the American League East. The Red Sox have the best record in baseball. Those 45 games constitute a baseball sample size too small for congratulations but too large for a fluke. This record puts the Red Sox on a pace to win 112 games. You can say that they might not win that many. But you can't say that they're not good.
That's the Yankees' problem. This is not the Boston club, its pitching running on fumes, which the Yankees pushed out of the way in a five-game Fenway sweep last August. This is not the Boston club that stumbled and fell into third place. This is a well-balanced team with admirable pitching depth.
The one element has separated the Yankees and the Red Sox, the one thing that typically made the difference between first place and second, was pitching. The Yankees typically have had that precious commodity in greater quality and quantity than the Sox. Not this time, at least not so far.
Entering Tuesday night, Boston was third in the AL in team ERA. New York was 11th. It is true that the Yankees have had numerous pitching injuries. And it is true that Roger Clemens will soon be riding in for the alleged rescue. But the Red Sox have seized the pitching high ground in this rivalry. The Yankees will now have to come and get them and the direction, even this early, is all uphill.
The Red Sox don't seem especially prone to slumps. They have not lost two in a row since April 24. No club is slump-proof, but with pitching strength and depth, the danger of a prolonged slide is seriously reduced.
When Red Sox manager Terry Francona was asked what he liked best about his club, his response was perfect: "It's something different every night, which is good."
Francona expanded that answer to include the notion that Boston's starting pitching had been consistent enough in working deep into games, that the bullpen was allowed to be as good as the Red Sox thought it was.
And the Red Sox have continued to win, even with Josh Beckett temporarily on the shelf. Their pitching will be even better when the story of Jon Lester's comeback gets the chapter that it deserves, with his successful return to the Red Sox rotation.
We are in uncharted waters for this rivalry. It is all smooth sailing for the Red Sox and stormy seas for the Yankees. Before this season, the Yankees had not previously been 10 games out of first place at any juncture during Joe Torre's tenure as manager. But again, half of this is a reflection on the Yankees' early struggles. The other half is a reflection of the stunning early success of the Red Sox.
The shoe has been on the other foot often enough. As the rivalry grinds onward, at least Francona has a brand new set of questions being posed to him in the Bronx. One of them was how it felt to be in Yankee Stadium leading the Yankees by such a wide margin.
"I don't know how to answer that," Francona said with a smile. "If this were Sept. 29, I'd say: 'It's awesome.'"
Francona was also asked variations on the theme of Boston realistically ending the Yankees chances of winning the division. The question seems a tad premature, and it is the sort of query that might normally sound like fingernails on a chalk board to the Boston manager. But here, in the new uncharted waters, he gave it a polite pass.
"I don't even have a thought on that," Francona said. "That hasn't even entered my mind. It has no bearing on our preparation for this series."
This line of questioning arose again during Francona's appearance on a New York radio sports talk show on Tuesday. Francona's response included multi-tasking.
"I did a crossword puzzle in the meantime," he said. "I think I'm getting OK about answering it without trying to get anybody mad. I'm getting to the point where I can do that and get 41-across at the same time."
In the past, the Yankees were the thorn that never left the Red Sox side, the rain on their parade, the plague of locusts on their harvest. Now the storyline seems to have taken a 180-degree turn. You could say it was a night-and-day difference, but that might be an understatement.
The Yankees have had some serious shortcomings of their own. But this season, there is also the very serious external threat to their perennial dominance of the AL East. That threat, that problem, would be the Boston Red Sox.
Mike Bauman is a national columnist for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.