Darren O’Day is a free agent. Ken Rosenthal reports on what’s going on with free agents. Mild controversy:

Sources: O’Day asking for four-year deal in $28M to $36M range. Most accomplished reliever on open market. — Ken Rosenthal (@Ken_Rosenthal) November 18, 2015

The argument against: as players go, Darren O’Day is relatively old, and old people wither and die. Also, O’Day is a reliever, and the perception is that relievers are made out of dry spaghetti noodles, subject to break with the lightest pressure. A four-year commitment to an aging reliever? No, sir. Better to find a starting pitcher, or maybe a young outfielder or something. Something you can set your watch to.

The argument for: Darren O’Day is really super good. The best predictor of someone being really super good next year is whether a player was really super good last year. Good players get big free-agent contracts. Really, that’s all you need to know. The rest of the post will only belabor this.

O’Day turned 33 a month ago. So, he’s looking for a guarantee through his age-36 season. That’s nothing weird. Ben Zobrist is 34, and he’s probably going to sign for four years. For some reason I just thought of James Shields, who signed through his age-36 season, with an option. Age is an important thing, but it’s less important than performance, and so let’s talk about O’Day’s performance.

The last four years, 239 pitchers have faced at least 1,000 batters. O’Day ranks tied for 8th in OPS allowed, even with Matt Harvey. He’s right in front of Joaquin Benoit. He’s the only pitcher of the top 14 to spend the whole time in the American League.

The last two years, 253 pitchers have faced at least 500 batters. O’Day ranks alone in 8th in OPS allowed, sandwiched by Pedro Strop and Hector Rondon. He’s no Wade Davis, but he’s ahead of Kelvin Herrera.

Generally, you don’t want to evaluate pitchers just by their overall results, or by their ERA, but O’Day is an exceptional case. Now, there’s nothing wrong with his peripherals. His FIP is perfectly strong. But his ERA is kind of insane. Going all the way back to 1900, nearly 2,400 pitchers have thrown at least 400 innings. O’Day has the very greatest difference between his ERA- and his FIP- (in the good direction), and though O’Day’s career isn’t over, so that could change, the idea is this: O’Day beats his peripherals. There’s something about him, and it’s hard to deny. It’s also easy enough to figure it has something to do with his weird low arm slot. Hitters hate him! One weird trick, and everything.

There’s something neat you can get from Baseball Savant. As a little bit of further evidence that O’Day is hard to hit well: last year, 288 pitchers allowed at least 100 batted balls picked up by Statcast. O’Day ranked fourth-best in average exit velocity. He ranked best by a good margin in average fly ball/line drive exit velocity. He allowed the lowest maximum exit velocity, at 105 miles per hour. It all counts for something. Hitters have trouble squaring O’Day up.

And he’s in no way showing his age. He hasn’t been on the disabled list since 2011. In 2014, he had his career-best velocity. In 2015, he had his second-best velocity. He’s up from where he was in his first year with the Orioles, when he was good, and he just posted a career-low contact rate. O’Day was good, then he started throwing harder, and now he’s more good. It’s hard to imagine a more encouraging sign.

What if we did this: I found 1,500 pitchers who’ve thrown at least 200 innings between the ages of 29 and 32, covering O’Day’s last four seasons. O’Day holds the fourth-lowest ERA-, six points ahead of fifth place. Joe Nathan and Mike Adams are tied for first in the pool, and then you find Jake Arrieta, whose numbers aren’t yet locked in because he’s got some years to go. O’Day has actually been historically effective, even if he isn’t as conspicuous about it as Aroldis Chapman.

For a quick little project, I put together some comps. Over the last three seasons, O’Day has been worth 7.4 RA9-WAR. That’s WAR based on ERA, instead of FIP. Between 1986 – 2011, I found 17 relievers who threw at least 150 innings between the ages of 30 – 32, with an ERA- no higher than 60. Over that span, they averaged an RA9-WAR of 7.3.

Between the ages of 33 – 36, those same relievers averaged an RA9-WAR of 4.0. Robb Nen got hurt and never pitched, but on the other end of things you have Mariano Rivera. Rivera might feel like a silly comp, but it’s not as insane as it sounds. Take O’Day. Say you project him at four years, and that quick and dirty 4.0 WAR. If you estimate $8 million per win, then that gives you four years and $32 million. That’s right in the very middle of the range Rosenthal reported. It makes perfect sense, then. If 4.0 WAR is a decent estimate of O’Day’s next four years, then $28 – 32 million is a solid expectation. You probably wouldn’t love O’Day at the end of the contract, but that’s how most free-agent contracts work. Value at the start, some dead money at the end.

And, you know, what are we supposed to make of O’Day’s injury risk? His stuff is actually playing up of late, and though he throws plenty of breaking balls, he also tops out in the mid- to high-80s. It’s speculation on my part, but if O’Day is less of a risk to break than your average slider-heavy reliever, then that only improves O’Day’s projection. Injuries destroy careers. If O’Day stays healthy, he’s already proven himself with a fastball at 85. Though he’s unconventional, so is Brad Ziegler, and he’s been just fine. That O’Day is unconventional might be exactly what makes him a strong free-agent investment.

It’s a good time for Darren O’Day to be a free agent. The whole industry just saw what the Royals did with a shutdown bullpen, so relievers are in particularly high demand. It’s also a good time to be a team buying Darren O’Day. It’s because he’s really super good. You could’ve quit reading this post a lot earlier.