PITTSBURGH -- For the first time this season, Mitch Keller finds himself in a bit of a funk.

Keller’s struggles continued as the Pirates lost to the A’s, 11-2, on Tuesday night at PNC Park. Across 5 1/3 innings, Keller allowed five runs (including a home run), walked four batters, recorded just one strikeout and generated a season-low three whiffs.

“I just didn’t execute,” Keller said. “It just comes down to execution. I left too many pitches over the middle, and they took advantage. Obviously, with the free passes, it was just not a good night.”

Keller is in the midst of one of the worst funks of his Major League career. In his past three starts, the right-hander has allowed 15 earned runs across 17 1/3 innings, raising his ERA from 2.44 to 3.60. There has been only one other instance in which Keller has allowed 15 earned runs in any three-start stretch: Aug. 18-28, 2019 -- those being the fifth, sixth and seventh starts of his Major League career. Earned runs, though, paint only one part of the picture.

While the earned runs have piled up, Keller’s three true outcomes haven’t been too out of whack. Across these 17 1/3 innings, Keller has totaled 17 strikeouts (8.8 K/9), seven walks (3.6 BB/9) and three homers (1.6 HR/9). On the season, Keller has a 10.58 K/9, a 2.36 BB/9 and a 0.90 HR/9. Keller’s numbers have trended downwards in his past couple starts, but he hasn’t completely lost his way.

“It felt like it was coming out really crisp and really clean, maybe too good,” Keller said. “Just too loose. I don’t know. I just didn’t have it.”

Each of Keller’s past three starts has been different compared to the last.

Against the Giants on Wednesday, Keller was on the wrong side of batted ball unluckiness, allowing three hits that registered an exit velocity of less than 70 mph, as well as several grounders that beat the shift. In the three true outcomes department, Keller struck out eight -- finishing the game with 14 whiffs (his season high is 15) -- walked one and didn’t allow a home run.

Against the A’s on Tuesday and the Mariners on May 26, by contrast, Keller conceded that he didn’t feel sharp. In Seattle, an unsharp Keller still fanned eight batters despite walking two and allowing six runs, including two home runs. Considering Keller has totaled the fourth-most strikeouts in the Majors (94), his one-strikeout performance against Oakland stands as an aberration.

Even with the recent struggles, Keller has been one of the Majors’ better starters this season, building off last season’s breakthrough. Through his first 10 starts, Keller owned a 2.44 ERA and a 2.68 FIP. In mid May, Keller won National League Player of the Week after tossing a shutout against the Rockies and following up that outing with a career-high 13 strikeouts against the Orioles.

With that success has come belief. Over the past year, Keller has discussed on many occasions the confidence that he feels when he steps on the mound now compared to his first couple seasons in the league. The past several weeks have not been friendly to Keller, but the 27-year-old won’t lose all the progress he’s made mentally because of three tough starts.

Prior to Tuesday’s loss, manager Derek Shelton discussed how his conversations with Keller have evolved over the past year. In the past, Keller might have focused his attention after a rough start on the quantity of runs that he allowed; now, Keller dedicates his energy to the process itself.

“Now, he does a much better consistent job of focusing on the process,” Shelton said pregame. “Sometimes you can't avoid it. Sometimes balls fall, bleeders fall in, swinging bunts, somebody makes an error, you hit a guy and then they hit a home run. There’s so many things, but I think he's focusing on the process now."