Twelve weeks into the season, the Dodgers are already turning to a 12th different starting pitcher in their revolving door of a rotation.

For Ben Casparius, it’s an opportunity he’s patiently waited for all season.

Though Casparius is technically still a rookie, the 26-year-old right-hander has seen a lot in his young MLB career. Last year, he went from starting the season as an overlooked double-A prospect (one who didn’t even get an invite to big-league spring training) to finishing it pitching key innings in four postseason games, including as an opener in Game 4 of the World Series.

This spring, a rash of bullpen injuries ensured he’d have a spot on the opening-day big-league roster. More injuries to top right-handed relievers pushed him into a de facto leverage role.

At every step, the former fifth-round draft pick has excelled, posting a 2.86 ERA over 44 innings this year with 46 strikeouts and only nine walks.

Along with fellow rookie reliever Jack Dreyer, Casparius has become one of the unsung heroes responsible for helping the first-place Dodgers overcome their injury-riddled start.

“We don’t win this game tonight without Ben,” is the kind of quote manager Dave Roberts has uttered more than once, and most recently after Casparius pitched 2 ⅔ scoreless innings of relief in a come-from-behind win against the New York Mets on June 3.

“He’s had to grow up really quickly for us,” Roberts added that night, “and he’s got the respect of his teammates.”

Now, however, Casparius is getting a new level of respect from the team’s decision-makers, too.

After exhausting virtually all their other starting pitching alternatives to this point — from struggling minor-league arms such as Landon Knack, Bobby Miller and Justin Wrobleski, to a bulk-inning option such as Matt Sauer — the Dodgers are finally entrusting Casparius with a starting role.

For all the value he brought in the bullpen, they simply couldn’t afford to keep him out of the rotation any longer.

“Where we were at [earlier this season], we felt that there was more value [having him come] out of the ‘pen and being kind of a versatile type reliever,” Roberts said. “But where we are at now currently, he’s certainly showing that he’s 1 of 5.”

Casparius got his first shot at a more traditional start on Wednesday in San Diego, producing four innings of one-run ball in an outing he didn’t know he was making until the night before.

In the days leading up, the Dodgers had lost Tony Gonsolin to an elbow problem — already their fourth starter to get injured just since the start of the season. They had demoted Knack to the minors, and watched Wrobleski give up four runs in six innings to the St. Louis Cardinals as his replacement. They saw Sauer get roughed up as a bulk-innings pitcher Tuesday against the Padres, and Miller implode in a 10-run outing in triple-A that same night.

Suddenly, the team was down to just three healthy starters it could trust: Yoshinobu Yamamoto, Dustin May and Clayton Kershaw.

Emmet Sheehan might be part of that group before long, continuing his recovery from Tommy John surgery with a third triple-A rehab start on Thursday in which he pitched 3 ⅓ innings (once he completes four innings, Roberts said, he will be a viable option for the big-league club). Shohei Ohtani, Tyler Glasnow and Blake Snell are also making progress towards returning, though none of them are likely to be back until sometime next month.

Emmet Sheehan, left, with Ben Casparius and River Ryan during the World Series ring ceremony in March, made his third triple-A rehab start on Thursday.

(Gina Ferazzi/Los Angeles Times)

Thus, with a Wednesday rubber match against the Padres looming, the Dodgers decided to reassign Casparius from multi-inning reliever to their latest fill-in starter.

“We like this kind of transition right now,” Roberts said. “Figuring out how we get through this period before we get other guys back to health … potentially there’s a chance to continue to build him up, which right now makes sense.”

In a win over the Padres that kept the Dodgers in sole possession of first place of the NL West entering another key series this weekend against the San Francisco Giants, Casparius responded with quality production. He limited damage (with the help of an Andy Pages outfield assist) to one run during a bases-loaded jam in the second. He retired the side in order in each of the other three innings he pitched.

Most notably, he also fought to take down an inning more than initially expected — lobbying to stay in the game for the fourth despite Roberts’ pregame assertion he likely wouldn’t pitch past the third (not since May 5 had Casparius thrown more than three innings in an outing).

“He wasn’t going to come out of that game after three,” Roberts said. “He wanted to stay in for the fourth.”

It gave Casparius the chance to flash his full arsenal of starting-caliber stuff; from a big-breaking combination of sweepers and curveballs, to a late-biting cutter that can induce soft contact, to an upper-90s mph fastball that, one point, even Padres star Manny Machado outwardly endorsed, pointing to Casparius with an approving nod of his head after swinging through a 98-mph heater up in the zone for a first-inning strikeout.

“I saw that,” Casparius said. “He’s one of the best players in the game, so it’s pretty cool.”

Casparius also showcased his evolved mental approach.

During his minor-league career, Casparius started in 57 of his 79 career appearances. Moving to the bullpen full-time at the start of his major league career gave him perspective he believes will benefit him in his return to a starting role now.

“Taking that reliever mindset, pitch by pitch, inning by inning, has helped me to slow the game down in general,” Casparius said. “So I think it’s been kind of a blessing. And then whatever happens going forward, I think I can just use that to keep going.”

Eventually, Casparius could be shifted to the bullpen again. Once the Dodgers get healthier, his value as a multi-inning relief option will likely mean resuming his swingman role.

But for now, Roberts has already confirmed that “the next time he’s on the mound, it will be as a starter.”

And for a pitcher who, despite his success out of the bullpen, has continued to view himself as a starter long-term, it represents an opportunity that might have been borne of out necessity, but was also long-ago earned.

“Obviously, I’ve been doing it for the majority of my professional career, so it’s something I’m comfortable with routine-wise,” Casparius said of starting games. “I’m just looking forward to what’s going on and what’s coming up next.”