PITTSBURGH -- There is never an ideal time to scratch a frontline pitcher from a start. However, Tuesday night seemed like a good time for the Padres to get away with it.

The Padres were facing the Pirates, who had managed a total of 30 runs in their past 13 games, losing 12 of them.

However, right-hander Reiss Knehr struggled as the Padres fell to the Pirates 9-4 in the opener of a three-game series at PNC Park. Called up from Triple-A El Paso on Sunday, Knehr made a spot start for Yu Darvish, who, suffering from an illness, did not join the Padres on their trip from San Diego to Pittsburgh.

Knehr lasted just two-plus innings and gave up five runs and six hits. He was lifted after allowing back-to-back home runs to Carlos Santana and Jack Suwinski to lead off the third inning, which gave the Pirates a 5-3 lead that they wouldn’t relinquish.

But Padres manager Bob Melvin absolved Knehr of most of the blame. Instead, he was disappointed that the offense produced only one run over the final seven innings after opening the scoring with a three-run second.

“We scored three runs and had good momentum, then they scored three and that shouldn’t have been it for us,” Melvin said. “It just seems like [the Pirates] had a little bit more spirit the rest of the game than we did.”

The Padres (37-42) lost their third straight game, continuing what’s been a tough season following a trip to the National League Championship Series last year.

“I would say typically, when you’re not scoring and not playing well, you look flat,” Melvin said. “When you score three runs in the second inning, you should feel good about yourself putting together good at-bats. And then just not have it the rest of the game, we got outplayed for sure.”

Star right fielder Fernando Tatis Jr. did not necessarily agree with Melvin’s assessment of the Padres being flat.

“They kept hitting and we didn’t. That’s all,” Tatis said. “We have been able to bounce back. It’s that simple. Everybody is playing with a lot of effort. We just didn’t score enough runs tonight.”

Early on, it didn’t seem that scoring was going to be a problem. Manny Machado led off the second inning with a double and came home on Gary Sánchez’s one-out base hit before Ha-Seong Kim capped the inning by hitting a two-run single with two outs. It was the second of three hits for Kim.

However, the Padres managed only one run after that, on Nelson Cruz’s RBI single in the sixth. That drew the Padres within 6-4, and soon after that, they had runners on second and third with two outs, representing the tying runs.

Trent Grisham, though, hit an inning-ending comebacker to Pirates starter Rich Hill. Roansy Contreras followed with three scoreless innings for his first professional save.

The loss to the slumping Pirates came after the Padres dropped back-to-back games over the weekend to the Nationals, who are in last place in the NL East. Prior to the current skid, the Padres had scored double-digit runs in back-to-back games.

The Padres haven’t won more than three games in a row all year.

“We’re not in the best place right now,” Melvin said. “If you had asked me three days ago, I would have said we were on the verge of being in a really good place. It’s been that way all year, to where it feels like we’re doing some good things, then all of a sudden, we have bad spurts right after that. You get tired of saying you can sustain something, but we haven’t done it this year.”