Say this about Trevor Rosenthal: At least his ERA is no longer infinity. (Patrick Mcdermott/Getty Images)

The two extremes of the Washington Nationals’ offseason — their best addition and, so far, their biggest problem — were on display Thursday afternoon at Nationals Park in a 4-2 win over San Francisco.

Or, rather, left-hander Patrick Corbin was on the mound, taking a one-hit shutout bid into the eighth inning. Reliever Trevor Rosenthal, signed to be the setup man, was not used again in the situations for which he was signed. His 40.50 ERA in six games, including eight walks, two hit batters and two wild pitches in just two innings, has made him the invisible man who must win back trust.

Corbin, who has a 2.36 ERA and a 33-5 strikeout-to-walk ratio in four starts, demonstrated just how much the Nationals have improved in several crucial areas since last season and how excellent they might be if they had a normal, functional bullpen, not a little shop of horrors lurking beyond their outfield fence.

In 7⅔ innings against the overmatched Giants, Corbin fanned nine, walked one and allowed only two hits and one run. His sliders, at varying speeds, were as merciless as expected for a pitcher with a $140 million price tag. His two-seam fastball at 92 mph also had Giants jumping back on strikes at their fists, and his curveball, one at 66 mph, left them off-balance.

Corbin’s range of performances, from solid to dominant, represents the best of the Nats’ offseason. Like the catching duo of Yan Gomes and Kurt Suzuki, with 13 RBI in 18 games, as well as solid starters Jeremy Hellickson and Aníbal Sánchez, Corbin has produced at least as expected, if not better.

There is, however, one huge unanswered Nats question: Can the camel pitch the eighth inning?

The Nats are not desperate enough — yet — to use a dromedary as a relief pitcher to get them over their late-inning hump. But all suggestions are welcome. The team’s 11.81 ERA in the eighth inning is the worst in the majors.

Whether night or day, the eighth inning comes and the Nats start to quake. The seventh inning has not been a problem, with the team’s 3.94 ERA in the top 10 in the majors. And when the Nats have a lead in the ninth, Sean Doolittle (0.91) has been the key to the team’s 9-8 start. But when the eighth inning arrives, you would have a better chance figuring out the Mueller Report’s redactions than guessing who or how the Nats will get three outs.

In their two wins over the Giants, the Nats had leads of 9-2 and 4-0 entering that nightmarish eighth inning, yet both times Doolittle, the bullpen’s only consistently trusted soul, had to be summoned to dodge a possible disaster — facing the tying run in the ninth inning in Wednesday’s game and the winning run in Thursday’s.

“You can’t be a hero. April isn’t the time to lay it on the line,” Doolittle said of the need to balance wins now vs. durability for the whole season.

Sometimes a big problem can have a simple solution. The key to such a revived bullpen, because the Lerner family doesn’t seem willing to sign free agent Craig Kimbrel, almost certainly will have to be Rosenthal.

In his first season back from Tommy John surgery, Rosenthal has consistently thrown his fastball from 98 to 101 mph and says his arm feels wonderful. His slider is as sharp as in his stellar years in St. Louis, when he saved 147 games, was an all-star and helped the Cardinals to a World Series.

He’s a friendly, broad-shouldered, open-faced and enthusiastic competitor. But he has often had control issues. In a way, that has made him the epitome of the “effectively wild” reliever — in other words, he’s terrifying. And playing for a new team and making a career comeback, he has been amped too high all season.

Knowing that you must calm down a notch and doing it are two different things. Especially when, in your first game as a Nat, you face four batters and get charged with four runs. Then you are removed from your next game after one pitch (a cheap hit). And in your next two zany appearances you face only two batters each time — resulting in three walks, a hit batter and two wild pitches.

That’s four games, no outs and an ERA, as many noted, of “infinity.”

The battle back has been hard. “It was nice to see Rosenthal smile after he came out of the game [Tuesday]. He’s getting close to being who he is,” Manager Dave Martinez said Thursday. “Look, I’ve seen Rosenthal pitch in the past. And he’s never been a guy to have clean innings every time. He’s going to walk guys. He’s usually in the strike zone. But when he’s not, he’s up and down, not east and west.

“We need to get him that way again. When he’s that way, he’s good.”

Martinez is bringing Rosenthal back slowly, perhaps even babying him more than necessary. “We’ve talked, and Trevor’s had slow starts in the past, but it falls into place for him,” Doolittle said. “In ‘Top Gun,’ Maverick has some doubts at one point. Jester says, ‘Keep sending him out there.’ ”

This column refuses to argue with unassailable movie-dialogue strategy. After watching improvements in Rosenthal’s two most recent appearances, I think he’s going to find himself and, eventually, be the key piece that solidifies the bullpen.

That is, if the Nats and Martinez have the grit to keep sending him out to do the job for which they hired him. That takes managerial guts.

“I’m ready now. I’m excited to get back out there,” Rosenthal said Thursday after he fanned two men in his inning Tuesday and only “yanking” one pitch to his glove side because he was over-amped and overthrowing. “I feel really good. I’ve been a little hyped, trying to do too much. I needed to get the jitters out. I have.”

The Nats’ bullpen has been so hideous — an 8.01 ERA even with Doolittle’s 0.91 included — that it has obscured every other positive development. If the Nats had one of their typical bullpens of the past eight years, they would have allowed about 23 fewer runs and have a fancy 99-64 run differential.

The next few weeks will show whether the Nats have a bullpen that will cripple their season and ruin a chance for a run at the National League East division flag.

Several pieces need to fall in place, but Rosenthal is the one essential. The Nats have nobody else who approaches his talent, experience or proven ceiling. Could his control problems be a tip-off to a career that’s unraveling at only 28?

Maybe. But probably not.

Either way, the Nats have no realistic choice but to suck it up, give him the ball when his role comes up — in that scary eighth inning — and find out the truth.