By Michael Hurley, CBS Boston

BOSTON (CBS) — Any team looking to sign Craig Kimbrel this offseason is going to have to spend a pretty penny — and perhaps shell out the most dollars ever for a closer.

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That has been believed to be the case for a while, but it seems to have been confirmed by Kimbrel’s agent.

According to Fancred’s Jon Heyman, agent Dave Meter is making quite the strong pitch to teams looking to sign his client. That pitch is:

He’s the best all time. Not just recently but all time. Lowest ERA, lowest batting average against, lowest OPS against. Also: he saved all his chances even this postseason.

That is … quite the pitch.

(Please take a moment to allow all Mariano Rivera fans to properly collect themselves. … OK, should be good now.)

We can start at the end, where Meter pointed out that Kimbrel was 6-for-6 in save opportunities this past postseason. Kimbrel did, technically, put forth a perfect postseason in that regard … but he also allowed 19 base runners in 10.2 innings, posting a 5.91 ERA. Had it not been for outstanding defensive plays by Eduardo Nunez (Game 4, ALDS) and Andrew Benintendi (Game 4, ALCS), that perfect stretch of save opportunities would have surely been broken. As it was, he still allowed at least one run in five of his nine appearances.

As for the rest of the pitch, it is certainly strong, but not entirely outrageous. Astute baseball observers have noted over the past two years that despite a somewhat quiet and unassuming demeanor for a closer, Kimbrel’s career trajectory has him headed for a Hall of Fame induction if he can maintain his production for roughly four more seasons. However you may believe Kimbrel has been as a closer, he’s almost assuredly been better than you think.

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Still, that particular Hall of Fame story was written in early July. That was before he hit a rough stretch, when he posted a 5.56 ERA and a 1.588 WHIP over 11 outings. He blew three saves in that span. He recovered with a strong September, before a nightmarish performance against Baltimore (of all teams) on Sept. 26. In that outing, he walked three batters and hit one more while allowing four earned runs and lasting just a third of an inning.

In terms of “best of all time,” we do have a neat and tidy comparable between Kimbrel and Rivera. Kimbrel has made 542 career appearances. Let’s compare that to the Rivera’s 567 relief appearances from 1996-2004 (we’ll eliminate Rivera’s 1995 season, when he split time as a starter and reliever).

KIMBREL (2010-18)
542 appearances
532.2 innings pitched
1.91 ERA
0.920 WHIP
868 strikeouts
14.7 K/9
4.23 K/BB ratio
333 saves
34 blown saves
89.8% save percentage

RIVERA (1996-2004)
567 appearances
661.1 IP
2.12 ERA
1.0222 WHIP
597 strikeouts
8.1 K/9
3.57 K/BB ratio
336 saves
47 blown saves
86.0% save percentage

Kimbrel clearly has been better than even Rivera to start his career, and thus the “best of all time” claims can be considered less preposterous than they may initially seem to some. (It must be noted, though, that in that same span, Rivera posted a 0.78 ERA and 0.755 WHIP over 67 postseason appearances, recording 32 saves and for holds while blowing five saves. Kimbrel, despite going 7-for-7 in playoff save opportunities, owns a 3.92 postseason ERA and a 1.258 postseason WHIP. Rivera allowed just two home runs in those 96 career postseason outings; Kimbrel allowed two home runs this postseason. October excellence does indeed matter in such comparisons.)

Overall for 2018, Kimbrel’s numbers were all right. But he posted his second-highest ERA, his second-worst strikeout-to-walk ratio, and the majority of his numbers ranked somewhere in the middle of his career stats.

In 2018, despite a bad postseason, Kimbrel was still one of the best closers in baseball. That’s been true over the past eight years. That much isn’t really up for debate.

The question that teams — teams that probably include the Red Sox, Braves, Cardinals, and some others — will have to answer is whether the 2018 season was a mere hiccup in an otherwise exceptional career, or whether it’s the first step toward a backward slide as the closer enters his early 30s.

It’s a question that’s not easy to answer. But at least we all know where his agent stands.

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You can email Michael Hurley or find him on Twitter @michaelFhurley.