Maybe we disagree on what an "ace" is, but to me, if you are top 30, you're close to an ace at worst. If you are top 15, there is little argument.
Archer's WAR rank in previous seasons:
11th in 2017 (4.6)
34th in 2016 (3.2)
10th in 2015 (5.2)
31st in 2014 (3.2)
2014-2017: 12th (16.2)
2015-2017: 8th (13.0)
(Better than Price, Verlander, Lester, Bumgarner, Keuchel & Cueto-- all considered "aces".)
The last 3 years, Archer has the 8th best xFIP at 77. He placed 29th in WHIP at 1.21. He placed 19th in K/BB at 3.80.
I'd say he's an ace.
His .325 BAbip this year is a major reason his numbers slipped.
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