"It's hard to say [why]," Chapman said through an interpreter. "I feel normal. If anything, it would be the rest, I guess. I think I had about a week without pitching. I can't really say why."
The outing was Chapman's first since serving up a go-ahead, 11th-inning homer to the Mariners' Yonder Alonso on Monday, and Chapman's first of at least one inning with no runs and no walks since Aug. 5 in Cleveland.
In Friday's appearance, Mookie Betts fought off a 103.1-mph fastball for a single, marking the fastest pitch that went for a hit in the Majors this season. The previous high had been the dramatic game-tying homer that Rafael Devers hit off Chapman on Aug. 13.
"He had a good inning. That's encouraging," Girardi said. "We'll continue to build on that. I think he's really important to us. I think we really need him. I was pleased with his inning. He looked like he got back into a count with a changeup. He got a strikeout with a slider. I thought he looked pretty good."
Girardi removed Chapman from the closer's role following a fourth straight ineffective appearance on Aug. 18 at Fenway Park, but Pedroia said that it was no picnic for the Sox to face the flame-thrower on Friday.
"He had some good stuff," Pedroia said. "That last pitch he threw me, that was nasty. It was like a 94-mph slider. Sometimes you just have to tip your hat and drag your bat back, I guess."
Friday marked Chapman's fourth appearance since being removed from the closer's role in favor of Dellin Betances and David Robertson, having entered in the sixth inning of games twice and once in the 11th inning.
The Yankees sent Chapman for a precautionary MRI on Sunday to investigate minor discomfort in his left elbow, but that exam came back clean.
"It's fine. I threw an inning today and had no issues at all," Chapman said.
Chapman had warmed up late in the Yankees' 6-2 victory over Boston on Thursday, and Girardi said that pitching coach Larry Rothschild had been working with Chapman on the grip of his fastball, trying to eliminate some of its cutting action.
"Maybe the work with Larry is what got him back on track," Girardi said. "Let's hope so, because again, I think down this stretch he is really, really important."