BOSTON — Aaron Boone isn’t ready to set up the Yankees’ postseason pitching just yet, but with just three weeks left in the regular season, some roles will soon start to take shape.

Luis Severino, rather than making his next start in the majors, will instead have a third minor league rehab outing, this time with Double-A Trenton on Wednesday.

And CC Sabathia, on the 10-day injured list with right knee inflammation, is set to throw another light bullpen on Monday.

If that goes well, Boone said the left-hander will start in Detroit on Wednesday — with Domingo German expected to follow Sabathia into the game.

It would be German’s second relief appearance of the season and would accomplish two things.

First, it would likely be the beginning of the Yankees limiting the right-hander’s innings, as he’s already set a career-high in that category. Secondly, coming out of the bullpen might also set German up to do the same in the playoffs if the Yankees decide to go that route.

The move isn’t unexpected, but it might be an adjustment for German, who has been the Yankees’ most reliable starting pitcher for much of the season.

So while the Yankees are closing in on their first AL East title since 2012, they still need to address several issues heading into the postseason.

Severino’s situation will be the one watched most closely.

There were expectations at the beginning of the season that he’d be the ace of the rotation and has instead missed the entire year with rotator cuff inflammation and then a lat strain.

If Severino returns and looks like his usual self, he could be a game-changer for the Yankees’ staff.

“I certainly like where he’s at,’’ said Boone, adding the Yankees preferred that he stayed in a controlled environment for one more outing.

Severino is scheduled to throw “60-plus” pitches on Wednesday, according to Boone, and then would head to The Bronx.

Deivi Garcia, the 20-year-old prospect who wowed the Yankees for much of the minor league season, is heading to Tampa, where he will continue to throw in case the Yankees decide to add him later this month.

For now, though, his season could be over.

As the regular season draws to a close, the Yankees will determine what their playoff rotation will be.

“Obviously, the hope is to win the division and have the days off [to set it up],’’ Boone said. “You start really shaping what you potentially have in October that final week.”

German has had success pitching in relief in the past — even this season.

After pitching well in two starts to open the year, German’s third appearance came out of the pen — in a 4-0 win started by Sabathia on April 13 against the White Sox.

After Sabathia tossed five one-hit inning, German struck out four in a two-inning outing before sliding back into the rotation, where he’s remained for the remainder of the season.

But with Severino possibly added to a rotation that figures to include Masahiro Tanaka, James Paxton and possibly Sabathia and J.A. Happ, the Yankees need to find a spot for German.

Boone hasn’t ruled out the possibility of using Sabathia in relief, either, although the 39-year-old has done that just once in his career. That was in the 2011 ALDS against Detroit.

“We may explore some different roles here and there with different guys,’’ Boone said. “But we’re not gonna map it out anytime soon.”