Note: this represents where the article stands relative to the Good Article criteria. Criteria marked are unassessed
Infobox and lede
"whom he helped win" → "with whom he won"
Very pedantic: should be a period after "Jr" in "Lew Jr."
Early life
Switch order of sentences so that the fastball is sandwiched between his high school career & Athletics signing
Philadelphia Athletics
Today in voices I hear: an undergraduate professor telling me never to start a sentence with "However". Here, especially, because some people read one section at a time and the part about his signing is in a different section, the construction of the first sentence jolts the reader
A little more awkward setup here: I think it can be made clearer that Mack was resting his pitchers so that they could be sharp for the Series and that this was still the regular season.
"third baseman/outfielder" → "third baseman and outfielder"
Cardinals and Red Sox organizations
No apostrophes needed in the header
Military and final season
"he was relying on curveballs to get hitters out" → "he was best known for his "assortment of curveballs"" (or something of that sort, I just really like the newspaper quote)