Jeffers and Kirilloff had similar BAs at AA last season and Jeffers has held his own pretty well. Sure Kirilloff's SLG was lower than Jeffers' was, but judging by his numbers from previous minor league seasons he definitely has pop. Jeffers also never played higher than AA.
Kirilloff played his last season less than a year removed from a hamate bone--an injury notoriously known for sapping power for a year post-surgery. This injury generally does not have a long-term effect on power potential, but in virtually all cases has a significant short-term effect even after the injury is healed and the player has returned to play.
Here's an article: https://www.beyondth...the-pitchfx-era
Here's the conclusion:
Overall, it appears that with time, player's who succumb to a hamate injury will find themselves back to previous power numbers upon their return to play, though it will probably take roughly a full season of at bats to do so. Of course a number of other factors could predispose a player to not bounce back quite as quickly or as robustly—age of injury, previous injury history, and even the time it took for correct diagnosis and treatment approach all play potential roles—but compared to other upper extremity injuries, the outlook for a full return to health and hitting power with the hamate fracture is generally promising.
Kirilloff's low-power numbers are not only not surprising, but expected. In addition, if you look at his second half splits versus his first half splits as he got further removed from the surgery... predictably his power numbers really began to improve. Kirlloff, as a hitter, is a couple of magnitudes better than Jeffers.