It may indeed be the future but it looks like the Astros' are running more of a 6-DAY rotation rather than 6-MAN. The team has 119 games and the top 4 starters have 21-23 starts, only 1-3 fewer than they would on a strict 5-man rotation through 119 games. I'd speculated on the Angels maybe wanting to try this so that keeping Ohtani fresh didn't necessarily take innings away from their other top starters.

For his 21 starts since his first one, Verlander has had 5 days rest before 13 of them, 4 days before 4 of them, 6 days rest between the first and second, then around the AS break rests of 7, 8 and 6. The point is that for many/most of those he got 5 days rest but the Astros only played 4 games on those 5 days -- i.e. the 5th/6th starter was skipped. If we expressed his off-days as games, we'd get:

4, 4, 5, 5, 4, 4, 5, 4, 4, 4, 3, 4, 4, 3, 7, 7, 4, 5, 5, 4, 5

So to a large extent, Verlander, Valdez, Urquidy and Garcia are on a regular 5-MAN rotation. Further, that seems to have been the plan originally. Odorizzi had just 12 starts through the end of July but that was mainly due to missing 6 weeks. He made 6 starts over 34 games after his first one before he missed time; then he came back as the 6th man for July with 4 starts in 24 games after his first July start. Cristian Javier began the year as a swingman, moving permanently into the rotation in mid-May, a couple of weeks before Odorizzi went down. After doing so, he also mostly got 4 games off each turn until recently.

They had a game every day from Apr 22 through May 8 (17 days) and used Javier twice. That was followed by a stretch of 16 days. Still they took advantage of the May 9 off-day to skip Javier, inserting him again for a start on May 14. Odorizzi gets hurt on May 16, they replace him with Javier and maintain a strict 5-man rotation through the rest of May and June.

Odorizzi returns in early July. The "problem" is that Javier has a 2.58 ERA (with the FIP to match) so he's not coming out of the rotation. So they do go to a 6-man rotation although Verlander misses a turn and there's the AS break. Odorizzi's last start for them was July 31, McCullers first start was Aug 13 -- he hasn't even had his 2nd start yet.

So the Astros were running a 5-man rotation, using a spot 6th man during long stretches with no days off. More precisely, when all 6 guys have been healthy, they have run a 6-DAY rotation, using the 6th starter during stretches without days off ... and other than the AS break, they had only one day off between June 28 and Aug 8 so Odorizzi's return was well-timed. Their 6th man became their 5th man (without replacing the #6 guy) when Odorizzi got hurt but he had the gall to put up a 2.50 ERA. So when Odorizzi came back, we don't know if they shifted to a 6-man rotation or stuck with a 6-day rotation, with no off-days, they are the same thing. They solved the question of "who's our #6" by trading Odorizzi, now they face the "problem" again with the return of McCullers. It remains to be seen how they'll deal with it -- as far as we know based on the actual usage data, they are still running a 6-day rotation though it's not clear whether McCullers or Garcia will be the 6th guy. They only have 5 days off the rest of the way and they may also want a 6-man rotation to keep everybody fresh.

A key point being that the use of a 6-man rotation has almost nothing to do with the Astros' 12.9 fWAR, 684 IP, 3.10 ERA and 6.2 WPA. But sure, the strict 5-man is probably on the way out. Teams will at a minimum use a 6th starter during extended stretches without a day off ... and for double-headers of course although those will probably disappear again once this season is over. I think they've been doing that for a while now, it's certainly not a new idea and it's easy enough to give a starter a seat on the AAA shuttle.

Longer-term ... if MLB actually sticks with the 13-pitcher limit then I'm not sure we will see 6-man rotations. We might if it actually increases SP innings so they can get by with 7 relievers (or they can use the AAA shuttle even more). Alternatively we see more multi-inning relievers -- which I think they are trying to crack. Still, I think we are more likely to continue to see 8-man bullpens (which may include a swingman) than we are 6-man rotations.

I think there will be a little bit of experimentation going forward, what the Astros are doing now, is pretty much what much of the teams did in the 50's and 60's with the "four" man rotation. Considering the number of teams that make the post season now, and yet the lack of need for a fourth or fifth guy in the post season, you might even see the Astros rotation used, with a opener in one or two of the slots. After all, a good opener would be a good post season reliever.

Sure, let's have six-man rotations.

Maybe seven or eight...hell, put the entire roster (including position players) in the rotation!

Put every baseball player ever into the rotation, even if they're dead!

Put EVERY HUMAN BEING THAT HAS EVER LIVED into the rotation! I WANNA PITCH IN THE BIG LEAGUES, COACH!

I have a hard time understanding why a starter who throws 70 pitches needs 6 days off but a relief pitcher who throws 35 pitches can throw every other day.

you know the conventional wisdom whenever the parade of relievers starts is: "if you keep changing pitchers eventually you'll pick the one that has nothing today."

Maybe it was never wisdom but more like a cliche or a meme or something. But I mean is that really a logical way to think? Like: "good pitching stops good hitting," or "he'll hit better in the 4 spot with some protection behind him," or 'He'll play better in a smaller town with less pressure."

Its obvious the trend in the past 20 years and probably the last 100 years has been: more pitchers on the roster and more pitchers entering ball games. So there's this total disconnect between a baseball cliche that everyone repeats and the clear evidence in front of us that nobody running baseball really believes this. Do they?

I have a hard time understanding why a starter who throws 70 pitches needs 6 days off but a relief pitcher who throws 35 pitches can throw every other day.

Fear. That's what it is, fear.

Starting pitchers are considered rare, fragile beasts, whilst relief pitchers are fungible, practically hanging off the trees. If you're a pitching coach and one of your relievers gets hurt, well, you can just create another one out of the muck, like they did with orcs in Lord of the Rings. (In fact, I think you can go to Wal Mart and buy Freeze-Dried Relievers (tm) by the dozen. Just thaw 'em out, and they can hit 95 on the gun! No muss, no fuss!)

But if you let a starting pitcher get hurt...oh mother mercy! You'll probably get fired and have to sell shoes in a distant (probably western or southern) state. And you will be shunned for all of your days, you monster.

Fear.

Contracts aside there doesn't seem to be much point in making any distinction between starters and relievers anymore. Manage your roster to optimize the use of your best pitchers and slot in everyone else around them as needed. Beyond that don't worry about roles and do whatever seems best for any game situation.

Within that context it would be great to see the return of the fireman role, that is the elite pitcher brought in to handle maximum-leverage situations. Leading 3-1 but the other team has the bases loaded with one out in the 7th? Time to bring in Gossage.

Remember when it was all the rage to advocate for returning to a 4-man rotation with a Sunday starter or swingman? That seemed to be almost consensus among sabr types about fifteen or twenty years ago.

This is much like what we saw when it shifted from 4 man to 5 man rotations. First a shift to 5 day, then to 5 man. One of the last was the Jays of 1982 where they had 3 guys with 38-40 starts each, a 4th with 23, then 5 more with 2-8 starts each. Dave Stieb (who should've won the Cy Young that year) had 17 starts on 3 days, 15 on 4 days, and 6 on 5+ days (including opening day). Did this hurt him? Nah. He was 24 and had 8 more seasons of 30+ starts each in him before his arm blew out. '82 was his first of 3 straight 7+ WAR seasons too. He also didn't go short in those games, his last 6 starts were complete games or extra innings (an 11 inning start where he got a ND)

To me, this is getting silly with 6 man rotations and the like. Ask most pitchers and I'd bet they'd LOVE to go back to 4 man rotations with the odd exception (guys like Ohtani for obvious reasons, and older pitchers who could do the old 'Sunday starter' thing that teams in the 50's did). The fear is crazy - guys like Alek Manoah could easily take on a bigger workload if built up to it. But instead we're likely to see less and less of the best pitchers in an attempt to 'maximize their utility' or somesuch. Teams need to recognize they get these guys for 6-7 years in the majors then they go away, and most will not last that long no matter how hard you try. So stretch them out and get those innings.

I think this is all of a piece with trying to reduce the value of any individual pitcher. Ideally, ownership would like all the pitchers to be Fungible Flame Throwers. A 6-man rotation, where eventually the "starters" may appear in a relief role mid-rotation, would help to affect this change. Ivy Leaguers thinking ahead. Doesn't matter to them whether it hurts the appeal.