Tanner Houck was like a yo-yo in 2021, bouncing between the majors and minors, and that’s an issue in baseball’s labor talks

Tanner Houck had a 1-5 record pitching for Boston last season, 0-2 pitching for Triple A Worcester.Jim Davis/Globe Staff

By virtually any measure, Tanner Houck was one of the best Red Sox pitchers of 2021.

Among the seven pitchers who made at least five starts for the Sox, Houck ranked second in ERA (3.52), first in expected ERA (3.20 as calculated by BaseballSavant.com), and first in strikeout percentage (30.5 percent). He combined nasty stuff with poise, maturity, and competitiveness, allowing him to be a consistent contributor whenever the Sox handed him the ball. He did not allow more than three earned runs in any start, whether in the second game of the regular season (5 innings, 2 earned runs, 8 strikeouts) or the second to last (five perfect innings and eight strikeouts against the Nationals).

Yet despite his contributions, Houck spent just 64 days in the big leagues while remaining on Triple A Worcester’s roster for 122 days. That service-time split meant Houck earned roughly $217,000 in the big leagues and about $58,000 in the minors — a total of roughly $275,000, or less than half of the major league minimum of $570,500.

That development was possible because teams can yo-yo young major leaguers on and off their big league roster — something the MLB Players Association and Major League Baseball have agreed to address in a new collective bargaining agreement.

Houck was a steady presence in the Red Sox transactions log. He was sent (at least on paper) to Worcester seven times, and promoted to the big leagues seven times. During one particularly absurd three-week stretch of July and August, the righthander stayed largely on turn for four starts in the Red Sox big league rotation while being sent back to the minors immediately after each.

Part of Houck’s limited service time owed to the fact that the Sox opened the year with five veterans in their rotation — with Nate Eovaldi, Nick Pivetta, Eduardo Rodriguez (after missing his first start of the year), Martín Pérez, and Garrett Richards remaining remarkably healthy. Once Rodriguez was ready to enter the rotation, Houck was sent to the minors to remain stretched out as a depth option.

Tanner Houck, seen here pitching during a July rain at Fenway, became a key part of the Red Sox staff in 2021.John Tlumacki/Globe Staff

Houck, meanwhile, was sidelined for roughly six weeks in May and June while on Worcester’s roster because of soreness in his flexor muscle.

When he returned to health, the Sox determined he was positioned to help in the second half, calling him up after the All-Star break. He contributed throughout the second half in the big leagues.

But the Sox shuffled him between Boston and Worcester repeatedly, often after spending just one day in the big leagues for a start. While players by rule have to remain in the minors for at least 10 days after being optioned, the Sox were able to work around that by calling up Houck as the 27th player on the roster for three doubleheaders — a role exempted from the 10-day option requirement.

The Sox had Houck pitch in the second game of a doubleheader on July 28, optioned him after the game; called him up for another doubleheader on Aug. 7, returned him to Triple A after the game; called him up on Aug. 12, moved him back to the Worcester roster immediately after the game; brought him up as the 27th man for a doubleheader against the Yankees on Aug. 17, returned him to Worcester after the game; and finally called him up for the rest of the season on Aug. 24.

Why all the shuttling? It’s all done in the name of roster flexibility. Because Houck was unavailable to pitch in the wake of a start, the Sox could add a bullpen arm to cover innings if he was off the roster, then bring him back for his next start.

Tanner Houck's roster flexibility afforded the Red Sox a chance to use him in many different ways.Jim Davis/Globe Staff

There are certainly baseball reasons for a team to manage its roster by maintaining a revolving door between the big leagues and minors. And the Sox were operating within their rights in bouncing the 25-year-old between their top minor league affiliate and the big leagues.

Once players are added to the 40-man roster, teams generally have three options on them — meaning three years in which they can be sent to the minor leagues without having to clear waivers. Yet within each of those years, under the collective bargaining agreement that expired at the end of the 2021 season, players can be shuttled an unlimited number of times between the big leagues and the minors.

What the Sox did with Houck is increasingly common throughout the game. Teams such as the Rays and Dodgers have long employed this sort of shuttle depth, particularly with pitchers.

One dramatic case involving both of those teams came in 2019. The Rays called up Casey Sadler six times in the first three months of the season, optioning him after the first five. On the sixth, they designated the pitcher for assignment, then sent him to the Dodgers in a trade. The Dodgers called up Stadler four times, optioning him the first three.

Like Tanner Houck, Casey Sadler is a pitcher who has been shuttled from spot to spot over the course of his career.Tony Avelar/Associated Press

Yet every demotion to the minors comes with financial and career consequences for players. Typically, young players have “split” contracts — meaning one salary in the big leagues, another in the minors. When they’re optioned to the minors, they receive their minor league salary instead of their big league salary. In Houck’s case, that meant earning just under $500 per day when in the minors, compared to just under $3,100 per day in the big leagues.

On top of that, players only accumulate big league service time while on active major league rosters. Player earnings are tied to service time, with the biggest raises coming when players have reached nearly three years of big league service time — when they qualify for salary arbitration — and six years of service time, when they’re eligible to become free agents.

With 64 days on the Red Sox’ big league roster in 2021, Houck has 100 days of total major league service time. Under the recently expired CBA, that amount would delay his arbitration eligibility until after at least the 2024 season. (Had he remained on the active roster for the entire second half, he might have reached arbitration after the 2023 season.) And he won’t be eligible for free agency until at least after the 2027 season — when the 2017 first-round pick will have played 11 professional seasons, and entering his age-32 season.

Both MLB and the Players Association have expressed openness to restricting such dizzying cycles between Triple A and the majors. In fact, the two sides had already done so. In 2019, in hopes of slowing the revolving door, the two parties agreed pitchers would have to stay in the minors for at least 15 days when optioned. But that rule alteration was suspended for 2020 and 2021 because of the pandemic.

Under the recently expired CBA, Tanner Houck won’t be eligible for free agency until at least after the 2027 season.Jim Davis/Globe Staff

Now, the sides are once again discussing ways of limiting the sort of roster tornado that swallows some optionable players. The MLBPA proposed limiting teams to using an option four times per year; MLB countered with five.

Of course, unless that restriction extends to call-ups of a 27th player for doubleheaders, it wouldn’t necessarily prevent a team from employing a Houck-like shuttle. Still, the exchange highlights at least one area where agreement between the two sides appears achievable.

The Players Association also proposed a change intended to end the option cycle earlier in careers — reversing a concession it made 15 years ago. Currently, players who are at least 19 when turning pro don’t have to be added to the 40-man roster (and thus subject to optioning) to protect them from the Rule 5 draft until after their fourth pro seasons; players who are 18 or younger when signing don’t have to be added until after their fifth season.

An industry source confirmed a report by Ken Rosenthal of The Athletic that the MLBPA proposed making unprotected players (those not on the 40-man) eligible for the Rule 5 draft after three pro seasons for those signed at age 19 and over and after four seasons for those who are younger. That was the system of Rule 5 eligibility through 2006, before the 2007-11 CBA prolonged that period by a year — a move that, for many players, likely delayed their eligibility for arbitration and free agency by a year.

MLB, according to the industry source, has not accepted that proposal, and it has been tabled for the time being — though it’s likely to be revisited at some point during the current talks.