SAN DIEGO — As far as Jon Lester is concerned, the biggest issue Monday night was that the Washington Nationals had a five-run lead that vanished, and vanished quickly, in an eventual 7-5 win over the San Diego Padres at Petco Park. Lester yielded three runs in an inning extended by Starlin Castro’s error and a searing homer to Fernando Tatís Jr., plus he left behind a runner on second that scored on a double off reliever Kyle McGowin.

He walked or plunked five of the 21 batters he faced. He lasted 3⅓ innings or 83 shaky pitches, however you want to slice it. Neither way is good.

“Just keep putting myself in bad situations, bad counts. I don’t have the stuff like I used to to pitch around those situations,” Lester said Monday. “So you have to kind of pick and choose your spots to try to fight back into some counts. I felt like I fell behind some of their big guys and didn’t want to give in and had some bases open, which then puts you in a bigger situation and when you do execute a pitch, you’ve thrown enough pitches where it doesn’t really matter.

“So I just continue to beat myself and hopefully I can make the adjustment and figure this stuff out.”

But the blown advantage doesn’t account for the ripple effects. By exiting in the fourth, Lester dropped another heavy workload on the Nationals’ bullpen. And by dropping another heavy workload on the Nationals’ bullpen, Lester tangled what was already a complicated pitching plan for Tuesday.

The Nationals entered this series without a designated starter for the second game. On Monday afternoon, Manager Dave Martinez hinted that they could use a group of relievers, beginning with Jefry Rodriguez. The idea was to give an extra day of rest to Patrick Corbin (who now will start Tuesday) and Max Scherzer (who now will pitch Wednesday), since July is always a solid time to do so. But Lester, the 37-year-old lefty, keeps shoving himself and his team to unfavorable positions. The only difference here was that the bullpen and offense picked him up to beat a very tough opponent.

After Lester was hooked Monday, McGowin threw 33 pitches in 1⅔ innings, Wander Suero threw 17 in a one-two-three sixth, Sam Clay threw six in a one-two-three seventh, Austin Voth threw six in a one-two-three eighth and Brad Hand threw 13 for the save, a day after he tossed 21 in a loss to the Los Angeles Dodgers.

The Nationals (41-42) could have still made Tuesday a bullpen game, but they would have had to account for some worn-down arms, a predicament that would have been softened by even a mediocre-to-solid start from Lester.

“I can tell you right now it will be a conversation tomorrow with the pitchers,” Martinez said Monday night of who will be available Tuesday. “Hopefully they’ll all be good to go. We got to make some roster moves, we haven’t decided yet.”

Lester did help build that early five-run lead. It began with Trea Turner, who homered on the first pitch he saw — a fastball from Padres starter Joe Musgrove — after missing five games with a swollen left middle finger. From there, Yan Gomes started the second with a walk, Victor Robles doubled, Lester singled in Gomes, Alcides Escobar executed a safety squeeze to score Robles, Turner singled, Juan Soto lifted a sacrifice fly to score Lester and Josh Bell singled to score Escobar. Then it was Bell who homered in the seventh, a right-handed shot off reliever Tim Hill, to give the Nationals an advantage that lasted.

First, though, Lester crumbled for a third consecutive outing. On June 25, he allowed seven runs on five hits in 2⅓ innings against the Miami Marlins. On June 30, he allowed five runs on seven hits in five innings against the Tampa Bay Rays. And on Monday, he loaded the bases in the first and escaped; logged a clean second; loaded the bases in the third, after Castro’s error, and hit one batter before Ha-Seong Kim ripped a two-run double down the left field line; then was on the wrong end of Tatís’s 116-mph homer in the fourth.

It was the hardest-hit ball of Tatís’s young career. It sounded like it, too.

“Hell if I knew, I wouldn’t do it, you know?” Lester said when asked if he has identified the problem. “I don’t know, maybe being too fine, a little too fine. I feel like I had a good feel of my change-up early on in the year and I just feel like I haven’t been trusting it a little bit. Maybe same thing with the sinker. I don’t know, I don’t know. If I knew, I wouldn’t constantly put myself in 2-0 situations to give these guys a chance.”

Lester lasted one more batter, yielding a double to Jake Cronenworth, and walked off with his eyes on the grass. The offense made it so the lapse didn’t hurt too bad. But it’s hard to count on Suero, Clay and Voth retiring the nine total batters they faced, in the same way it’s hard to count on a homer and single from Turner, two doubles from Robles, two singles and a homer for Bell, and an RBI double, two singles and that safety squeeze from Escobar, all in the same game.

For most of this season, his first with the Nationals, Lester has slimmed the margin for error and required that kind of effort from everyone else. His high-80s fastball has been punished by opponents. His curveball and change-up have been inconsistent at best. His cutter, often his go-to pitch, is exposed when few or no other options are working.

Once Stephen Strasburg or Erick Fedde return from the injured list, there will be a case to keep Paolo Espino in the rotation and put Lester elsewhere. And if Strasburg and Fedde are both healthy in 2021, there would be an even stronger case to fill the rotation with Scherzer, Strasburg, Corbin, Fedde and Joe Ross, squeezing Lester out.