“If you want to be a World Series team, you’ve got to consistently do the little things,” Britton said. “And that’s what they did last year. That’s all facets of the game: not walking guys, getting ahead, making good pitches all the way through, moving guys over, playing good defense, not giving up extra outs.

“That’s the separator between us and them on a given night, and the team that consistently does that is going to be the team that season series-wise is the winner.”

The Yankees hold the most decided edge over the Red Sox in the bullpen, even without the injured four-time All-Star Dellin Betances, thanks to relief pitchers like Aroldis Chapman, Adam Ottavino, Tommy Kahnle and Britton. But the unit has sputtered more in recent weeks, perhaps a domino effect from the rotation’s regular inability to pitch deep into games. As a result, the Yankees have also been scouring the relief market, where there are more options available. They recognize the importance of a dominant bullpen come October and hope Betances will have returned from his shoulder and latissimus dorsi injuries by then.

While the postseason is still more than two months away, the lack of an ace has been apparent for the Yankees. Luis Severino, the team’s best starting pitcher, hasn’t thrown a pitch all season because of a rotator cuff injury and a puzzling lat strain. (He recently resumed throwing and is racing against the calendar to build up his workload to return as a starting pitcher. If not, he might be an option in the bullpen.)

Domingo German has pitched like the Yankees’ best option for an ace, particularly on Sunday. He allowed three runs and struck out nine batters over five and one-third innings to improve to 13-2 on the season. He was the first Yankees starting pitcher since July 20 to complete five innings and the only one with an adjusted E.R.A. above league average. The Red Sox have four such starting pitchers.

“I felt like it was my responsibility to go out there and put a stop” to the losses, German said.

The Yankees’ biggest off-season acquisition for the rotation, James Paxton, has the talent to pitch like an ace despite past questions about his durability, and he has struggled to the tune of a 4.72 E.R.A. this season and dealt with a knee injury. Masahiro Tanaka (4.79 E.R.A.) and J.A. Happ (5.23 E.R.A.) have remained healthy but haven’t been consistent on the mound. The Yankees knew C. C. Sabathia (4.78 E.R.A.) would need to manage his chronically balky right knee and, on Sunday, it landed him on the injured list for the third time this season.