The Seager sibling rivalry

Four years and a handful of days after Kyle Seager’€™s promotion to the big leagues and seven months after the Mariners made him a nine-figure player, there soon will be another Seager on the way.

Corey Seager is the No. 1 prospect left in all of minor-league baseball according to Baseball Prospectus’€™ Midseason Top 50. The word "left"€ almost has to be in that sentence given that his ranking is due in some part to the cavalcade of call-ups who enlivened the first half of the season. But he’€™s not just a default guy. As a 21-year-old and predominantly as a shortstop, Corey is hitting .308/.356/.520 between 20 games in Double-A and 63 at Triple-A, making him a top prospect candidate in even the strongest classes and a candidate to form the best brother-brother duo of our era.

Since the decline of Melvin Upton and phasing out of the various B-list Molinas, the top spot on baseball’€™s brotherly duos list has been there for the taking.

So the question is now: Who’€™s Hank and who’€™s Tommie? OK, too extreme. Who’€™s Jason and who’€™s Jeremy? Fine. Who’€™s Roberto and who’€™s Sandy? In other words, what are the chances that Corey Seager eclipses his older brother and has the better career?

As the older of two sons, here’€™s where I’€™m supposed to say that the chances the little brother will ever be better are zero, but let’€™s aim for something at least a little bit more methodical.

Big brother Kyle is halfway through his age-27 season, and adjusting his stats for the difficulty of his home park, he’€™s amassed 18.4 wins above replacement player. The PECOTA projection system pegs him for 1.7 WARP in the Mariners’€™ remaining 73 games, so let’€™s say for this exercise that Kyle has 20.1 WARP at the conclusion of his age-27 season.

He’€™s not a super-duper-star, but that’€™s still a difficult plateau to reach for anybody. And let’s begin right there: with just anybody. Starting the probabilities at their simplest level, we can find the chances that any player to debut in the big leagues –€“ which we assume Corey will at worst by roster expansion day in September –€“ reaches 20.1 WARP by the end of his age-27 season.

Since 1950 –€“ the beginning of the era in which WARP has been calculated (or retro-calculated) –€“ there have been more than 5,000 position player debuts. Removing players who have not finished their age-27 season and have been active in the last two years (a rough proxy for "€œstill having a chance" to add to their total), there are 4,789, and of those, only 161 have reached 20.1 WARP by Kyle’€™s age. So the extremely rough baseline here is 3.4 percent.

Little brother has a little head start, though. (Don’€™t they always?)

While Kyle got four years and a partial to amass his age-27 cumulative stats, Corey could get as many as six and a partial if he comes up this year and doesn’€™t get injured or long-term demoted. Start to parse debut years/months, and you’€™re making for small sample sizes and mixing in a confounding but important variable of prospect status that we’€™ll get to later.

But taking a longer-term view will be important. The difference between the four years and the six years is vital, but if we look at a more complete career, the effect of different debut ages is tempered a bit. PECOTA projects Kyle Seager’€™s performance all the way out to 2024 –€“ three years after his Mariners contract ends –€“ and he’€™s projected for 39.2 WARP.

Out of 4,403 eligible position players since 1950, 162 have reached that WARP number by age 36. (It’s comforting to see in a projection system that it’€™s almost the exact same number reaching Kyle Seager’€™s real stats by his real age and his projected stats by his projected age.)

Corey Seager, though, isn’€™t just any player and thus isn’€™t a 162-in-4,403 (3.7 percent) shot to eclipse his brother by his mid-30s. He’€™s a top prospect in baseball, and in that fact alone, there is additional information.

For a longer history of prospect rankings to give us some more data, we turned to our friends at Baseball America, who have compiled their preseason top-100 lists since 1990. After scrubbing the repeats, the pitchers and those who are still compiling, even looking at the top 10 prospects makes this look like far from a sure thing.

Top position prospects, age 36+ or retired, who beat Kyle Seager’€™s age-36 projection:

Chasing Kyle Seager

  Players ≥ 39.2 WARP Total players Percent ≥ 39.2 WARP
Top 10 prospects only 16 64 25.0%
Top 20 prospects only 24 110 21.8%
Top 50 prospects only 32 244 13.1%
Top 100 prospects 33 407 8.1%
All players 162 4,403 3.7%

Kyle Seager was never a top-100 prospect, but just by actualizing what he has done in his first four seasons, he’€™s turned the average top-10 prospect — Corey was No. 5 preseason at BA and No. 7 at BP –€“ into about a 1-in-4 shot to beat what he’€™s projected to do.

For what it’€™s worth, PECOTA has Corey Seager reaching 9.2 WARP by age 27 based on comparable minor leaguers, but there is a ton of variance in a projection off zero major-league at-bats.

What’€™s more important for the Dodgers, who enter the season’s second half 4.5 games clear of the Giants in the National League West, is not 15 years compared to Kyle Seager, but 11 weeks compared to Jimmy Rollins.

As the BP Visual Year-to-Date stats show, not only is shortstop the position at which the Dodgers have the lowest production this year at 0.2 WARP, but that figure is also the lowest for any National League team’€™s shortstops.

Rollins has started 79 of the Dodgers’€™ 90 games and entered the All-Star break with his first 18-starts-in-19-games stretch. PECOTA prefers Seager on offense, but the two have essentially the same median forecast for the rest of the season –€“ both 0.6-win players. Seager’€™s forecast, as you would expect with a rookie vs. an established veteran, has a much wider range, and maybe that’€™s not what a team that’€™s trying to protect a lead needs to be going for.

Seager should get his chance soon enough, even if it’€™s not to replace Rollins but to take some at-bats at shortstop and third base down the stretch with expanded rosters. And then the brotherly conquering –€“ and competition –€“ can begin.

(Thanks to Rob McQuown of Baseball Prospectus for research assistance.)

This column was written by Baseball Prospectus author Zachary Levine.