PITTSBURGH -- On the surface, Mitch Keller looked in position to throw another inning. Through six frames, Keller had thrown just 84 pitches. The right-hander had pitched well, too, allowing just two runs, striking out eight and walking none. Instead, manager Derek Shelton went to his bullpen for the final three innings -- a decision that proved costly.

Following the Pirates’ 4-3 loss to the Diamondbacks on Saturday at PNC Park, a game decided by Pavin Smith’s two-run homer off reliever Robert Stephenson in the seventh, Shelton expounded upon his decision to pull Keller despite the right-hander’s low pitch count.

“In the sixth inning there, he kind of lost his delivery a little bit,” Shelton said. “We saw the pitches start to not have the same action. As well as he’s pitched over the last two starts -- and even today -- he hadn’t been in the stretch. Once he got to the stretch, he kind of lost where his arm slot was and the number of pitches he threw in that inning.”

Keller, as Shelton mentioned, did not pitch out of the stretch for nearly his entire outing. The Pirates’ ace struck out Josh Rojas to begin his afternoon; allowed a solo home run to the second batter of the game in Ketel Marte; then retired the next 16 batters he faced. It wasn’t until Keller plunked Rojas with two outs in the sixth inning that he had to pitch out of the stretch. Once in the stretch, Keller couldn’t match the dominance he had through the game’s first five innings.

After hitting Rojas, Keller allowed a single to Marte. Rojas and Marte executed a double steal, putting runners on second and third, then Corbin Carroll drove in Rojas with an infield single. Shortstop Tucupita Marcano prevented Carroll from tying the game, diving to knock down Carroll’s grounder up the middle.

While Keller had only thrown 84 pitches through six innings, seven of his last 13 pitches in the sixth inning were out of the strike zone. Keller had averaged about 97 pitches per outing in his first nine starts of the season, throwing fewer than 90 pitches only on one occasion, but given Keller’s lack of feel, Shelton elected to go to his leverage relievers to finish the ballgame.

Shelton noted that Keller’s workload this season didn’t factor into his decision.

“Once he got into the stretch he didn’t look like the same guy,” Shelton said. “He gave up the solo homer and didn’t walk a guy, so he had not been in the stretch. Once he gave up the base hit, he didn’t look like he had the same stuff. Having a full bullpen and having [Stephenson], [Colin Holderman] and [David Bednar] available, we made the decision to make that move.”

Keller, who took a no-decision in the defeat, echoed Shelton’s sentiments. “I hadn’t been in the stretch all game,” he said. “That was foreign territory for me there. I didn’t feel too sharp out of the stretch there, but we got out of it.”

While Keller’s evening ended relatively early from a pitch count perspective, Shelton had a fresh bullpen at his disposal for the final three innings. While Holderman pitched on Friday, Stephenson hadn’t pitched since May 17 and Bednar hadn’t pitched since May 14. Given how well the backend of Pittsburgh’s bullpen has pitched this season, the Pirates were in a good spot to secure the win.

Stephenson has been a reliable higher-leverage reliever for the Pirates this season, entering Saturday’s game with a 1.64 ERA across 11 innings, but the right-hander allowed the go-ahead, two-run home run to Smith that decided the ballgame. The location wasn’t poor -- Smith had a career .163 batting average on pitches down-and-in entering play -- but Smith managed to put bat-to-barrel and give the Diamondbacks a one-run advantage.

The Pirates had an opportunity to tie the game in the eighth inning after Jack Suwinski hit a one-out double, but Carlos Santana grounded out and Ke'Bryan Hayes, who hit a bases-clearing triple in the third inning, popped out to end the threat.

“We cannot stand on the fact that we get the one two-out hit,” Shelton said. “We have to continue to go and we didn’t do that today.”