How many innings do you expect a high-end reliever to accumulate over a five-game period? Traditional relievers top out at around 75 IP based on 2019 numbers, so that math would work out to 2.1 IP every five games. Two to three innings as over the course of a typical five-man rotation turning over. This initial question helps us understand how valuable an individual roster spot is in terms of usage.
It is no secret that starters share of innings pitched in baseball continue to decrease each year. However, a combination of the three-batter minimum killing off the ROOGY and LOOGY relivers and the eventual 13 pitcher roster limit, will mean that teams will need to get more outs from roster spots typically assumed to belong to relief pitchers. In the pursuit of flexibility and maximizing innings per roster spot, the Rays have seemingly found a multifaceted solution. An "Accordion" Rotation.
The Rays have carried as many as seven pitchers traditionally referred to as "starters" on the roster at once, yet their rotation rarely takes more than five games to turnover. The Rays have typically had a number of their starters "piggyback" each other to build up innings, either to open the season after Spring Training, get a younger pitcher stretched out, or rehab a pitcher back from injury. However, this strategy has been so effective, the Rays should embrace it permanently even after these players are stretched out and healthy.
This "Accordion" Rotation allows the Rays to flex these pitchers and mix up which days some of these pitchers see the mound. They can stretch out the rotation to a six-man rotation if they need to give starters locked into a traditional role more days rest. Or they can collapse it and use two of these pitchers in bulk relief to go 3+ IP. Since a bullpen roster spot is only worth 2.1 IP over 5 games, getting 3 IP from any potential bulk reliever over a five game span is actually adding value compared to a bullpen spot.
Tyler Glasnow, who has 62.0 IP in 10 games, is the only labeled starter who has not contributed to a "piggyback", and there should be no need. He has dominated the league since adding a slider as an effective mix up to his feared fastball and curveball.
But for Ryan Yarbrough (46.2 IP), Rich Hill (44.0 IP), Josh Fleming (40.2 IP), Michael Wacha (30.1 IP), Shane McClanahan (22.1 IP), and Luis Patino (15.0 IP), the Rays can mix-and-match using them as starters and bulk relievers.
To extend the exercise further, the average MLB team pitches around 1450 innings over a 162-game year, so 13 roster spots need to produce 112 IP each to meet that demand. For each reliever a roster carries, it means the share of innings pitched starters/bulk pitchers need to produce increases. If the roster has 6 dedicated reliever spots getting 75 IP, that would account for 450 innings.
This means the other 7 need to carry the next 1000 of the workload. With one starter getting 200 innings (Lets pencil in Glasnow for that), it becomes very viable to have 6 other starter/bulk pitchers live in the 133 IP range. Ryan Yarbrough pitched 140+ innings in 2018 and 2019 in a role that already reflects how this strategy treats pitchers. It is also worth noting that Yarbrough had been optioned to Durham for over a month in 2019.
In order to reach that 133 IP range, the magic number becomes 4 IP per turnover. Having a traditional start of 5 IP, and then a relief outing of 3 IP makes the math very attainable. While youngsters like Patino and McClanahan will be handled with kid gloves and may reach around 110 innings; Hill, Yarbrough, and Fleming seem well equipped to make up the difference with around 150 IP each.
Extending the rotation to six may hinder the math a bit. Multiple players will be shuttled in back and forth from Durham and the Injured List to make the staff have fresh arms from day-to-day.
Even if the Rays were to go back to a traditional 5-man rotation, keeping two starters on the roster to have scheduled "piggy-backs" of 3 innings again proves valuable due to the 2.1 IP rule. Young players like a Luis Patino can contribute to the team only pitching 3 IP per appearance.
These two outcomes do not even need to occur in the same game. It also means the team can survive "opener/bulk-guy" outings where Yarbrough or Fleming only go 4 IP. The team just needs to be able to get usage out of all 7 starters on the roster.
To sum things up: The Accordion Rotation is a method the Rays have seemingly embraced to carry around seven starting pitchers to eat innings more efficiently than what a traditional reliever should be expected to get, even if the starters are pitching less innings overall. The team can contract these seven into a five-game rotation and use them as bulk relievers, or extend the rotation out to six or even seven games (often to help with injuries or lack of off-days.) If one starter can soak up 200 innings, the rest only need to account for 4 innings per rotation turnover to meet the demand of innings. A bullpen spot is only worth 2.1 IP per rotation turnover, so getting 3 IP per rotation turnover is still a valuable use of a roster spot.
While this strategy may help the Rays transition young players to become MLB ready and ease unheralded veterans into more favorable situations, the strategy as a whole may transition baseball into a new era of collaborative pitching.