Number 4: Joe Nathan
| Pitcher | ERA | FIP- | WHIP | IP | K% | BB% | WPA/LI | fWAR |
| Joe Nathan age 28-30 | 2.44 | 63 | 1.01 | 221.1 | 30.4 | 8.9 | 5.67 | 6.7 |
| Craig Kimbrel age 28-30 | 2.44 | 57 | 0.91 | 184.1 | 42.3 | 10.4 | 5.14 | 6.0 |
| Difference in Performance (Kimbrel – Nathan) | +0 | -6 | -0.10 | -37 | +11.9 | +1.5 | -0.53 | -0.7 |
| Joe Nathan age 31-35 | 1.73 | 59 | 0.91 | 276.1 | 31.1 | 7 | 7.65 | 8.7 |
If Dotel represents the floor for Kimbrel’s future value, Joe Nathan has to represent the ceiling. Nathan’s performance was nearly identical to Kimbrel’s in their age-28 to age-30 seasons. The biggest distinction between the two is their K%, but this is largely attributable to the difference in the era that these two pitchers played in (Nathan was in the top 10 for relievers in K% from 2003-2005). Nathan, like Kimbrel was undeniably an elite reliever throughout his late twenties, but once he reached the other side of thirty he upped his game even further and cemented himself as one of the greatest relief pitchers of all time.
The bad news for those hoping Kimbrel delivers a similar performance is that Nathan’s later success was driven by his ability to keep his walk rate down as his stuff inevitably declined with age. Though their walk rates weren’t too far off during their respective age-28 to 30 seasons, Nathan was able to improve his walk rate to a near-elite level after his 31st birthday. Between 2006 and 2009, Nathan’s walked rate stayed below 7% every season.
It’s fair to say maybe Kimbrel can improve his walk rate as well, but there’s no reason to expect this type of drastic change. In Nathan’s case, his command had already been trending positively for years. He lowered his walk rate every season from 2003 to 2005, giving up free passes to 10.4%, 8.1%, and 8.0% of batters in his age-28 to age-30 seasons respectively. In contrast, Kimbrel walked 13.6%, 5.5%, and 12.6% batters in 2016, 2017, and 2018 respectively. That certainly isn’t a pattern that makes you think Kimbrel is going to make similar strides in his command.
Despite this, however, Nathan is still a very strong comp for Kimbrel. In every area except for free passes, it’s hard to separate the performances of these two pitchers from age 28 to 30. Kimbrel almost certainly won’t reach the level of excellence that Nathan achieved, but it wouldn’t be unreasonable to hope for Joe Nathan with a few more walks. If the Red Sox get a Nathan lite type performance over the next five years they would certainly be thrilled.