The 25-player rosters that are in effect from Opening Day to Sept. 1 are in more flux today than could have been imagined as recently as a half-dozen years ago. First came 13 pitchers and eight-man bullpens, and then came the demand that not only should there be eight of those arms but that they must be constantly refreshed.

The surest way for a reliever on the margin to get sent back quickly to the minors is to pitch two or more innings, and with little consideration as to whether those innings were good or bad.

A review of the 30 rosters early Friday indicated there were eight teams carrying 12 pitchers: the Mets, Washington, Milwaukee, Pittsburgh, St. Louis and Arizona in the National League, and Toronto and Houston in the American League.

The Twins joined the large 13-pitcher club Wednesday, and they stayed there by replacing spot starter Kohl Stewart (returned to Class AAA Rochester) with reliever Matt Magill on Friday.

Jake Cave, the fourth outfielder, was optioned to Rochester to make space for the 13th pitcher. And it appears the Twins are going to stay this way for a while, making them the leanest they have been in the outfield since 1975, when owner Calvin Griffith allowed Steve Brye to occupy a roster spot for several weeks with a cast on a broken hand.

OK, that was a poor choice of adjectives. The superlative of “lean’’ doesn’t apply when the fourth outfielder is now a 5-8, 240-pounder who carries the nickname La Tortuga — as in, The Turtle.

That would be Willians Astudillo, honored with T-shirts on Friday night but not in manager Rocco Baldelli’s starting lineup. “How can you not play La Tortuga on La Tortuga Night?” was the cry of Twins fans on Twitter, etc.

This is why: He’s now the fourth outfielder more than the third catcher, and the real outfielders are Eddie Rosario, Byron Buxton and Max Kepler. Add the designated hitter in Nelson Cruz and you want those key pieces in the lineup.

This was the Twins’ 23rd game, with Jason Castro as the starting catcher for 10, Mitch Garver for nine and Astudillo for four.

At this point, the Twins and Arizona are the only teams carrying three catchers. There’s this difference:

Arizona has 12 pitchers and no DH, allowing it to fit two extra catchers into a five-player bench. The Twins now have a three-player bench, and on Friday, it was Astudillo, Castro and infielder Ehire Adrianza.

Asked about the contrast with Arizona, Baldelli said: “We have Cruz who takes up a spot for other guys as our DH, but what we’re gaining with Nellie is huge. So this is the way we operate.

“We have one of our catchers who can be thought of as a multi-positional player who also happens to be a pretty good catcher, too. You could look at our roster and say it’s different. There are also some definite strengths that allow us to function in ways that others function.

“In general, having three catchers limits the times any of those guys is going to start. Is that to the detriment of any of them? No. I think maybe getting extra time can potentially help those guys, too.

“None of them is going to catch 110, 120 games. … That can be a strength for us.’’

Bill Evers is the veteran baseball man whom Baldelli brought in from Tampa Bay as a coach. One of his duties is as the liaison to the catchers. He never had three during a decade managing the Durham Bulls, but he sees three working here — and with small attention to the starting pitchers.

“This isn’t like Steve Carlton only threw to Tim McCarver,’’ Evers said. “Rocco fills out the lineup card and our catchers work with whatever starter is next in line. Castro is an outstanding veteran back there, and Garver has made a nice adjustment in his set-up behind the plate, and Astudillo … he has wonderful hands as a catcher.’’

And also this, as Baldelli said: “Tortuga is just a really good baseball player. And I’ve seen good athletes in all shapes and sizes.’’

Barring injuries, this appears to be the way the Twins will go with their 12 position players until Miguel Sano — now playing in extended spring training in Fort Myers — is ready to join the Twins and takes over at third base.

Then, our guy La Tortuga will have another roster complication to survive.