Number 3: David Robertson
| Pitcher | ERA | FIP- | WHIP | IP | K% | BB% | WPA/LI | fWAR |
| David Robertson age 28-30 | 2.83 | 64 | 1.01 | 194 | 33.6 | 7.0 | 3.93 | 5 |
| Craig Kimbrel age 28-30 | 2.44 | 57 | 0.91 | 184.1 | 42.3 | 10.4 | 5.14 | 6.0 |
| Difference in Performance (Kimbrel – Robertson) | -0.39 | -7 | -0.10 | -9.9 | +8.7 | +3.4 | +1.21 | +1.0 |
| David Robertson age 31-33 | 3.27 | 81 | 1.1 | 124 | 31.2 | 10.7 | 2.27 | 2 |
It’s a bit ironic that David Robertson shows up on this list given that the Red Sox were interested in him this offseason before he inked a two-year pact with the Phillies. Considering how well Robertson has held up with age and how much less of a commitment he received, it makes one wonder whether he was the reliever to get this offseason instead of Kimbrel.
To be fair, it’s not that simple. Robertson will be 34-years old next season and even at his peak one would be hard pressed to argue that he was ever as good as Kimbrel is now. As you can see above, Kimbrel has been better in basically every area besides walk rate over the same ages (this is a pattern you’ll find comparing Kimbrel to most elite relievers as it’s hard to be as dominant as he has been while walking 13% of hitters).
But this exercise should make us all appreciate just how good David Robertson is. His performance during his age-28 to age-30 seasons was just a step below Kimbrel’s and he has maintained that performance in his three subsequent seasons. If the Sox re-sign Kimbrel and he holds up as well as Robertson has, they would have nothing to complain about.