MacKinnon, 27, is a former 32nd-round draft pick who made his big league debut this season after a huge showing in Triple-A Salt Lake. He went just 7-for-37 at the MLB level without an extra-base hit at the Major League level, but the righty-swinging MacKinnon mashed at a .324/.429/.631 clip in 273 plate appearances with Salt Lake (156 wRC+). In just 273 plate appearances in Triple-A, MacKinnon belted 14 homers and connected on 19 doubles and four triples — all while walking nearly as often as he struck out (15.4% to 18.7%).
Because MacKinnon was only selected to the Major League roster for the first time this season, he’s in the first of three minor league option years. And while he’s never quite produced at this level in the minors in the past, MacKinnon still slashed .285/.380/.474 (139 wRC+) with 13 dingers and 30 doubles through 426 plate appearances in a pitcher-friendly Double-A setting a year ago.
Following the offseason trade of Matt Olson, the A’s have given most of the playing time at first base to lefty-swinging Seth Brown and the since-traded, right-handed-hitting Christian Bethancourt. Brown has batted .249/.310/.494 when facing right-handed pitching but just .170/.200/.283 in 55 plate appearances against lefties. MacKinnon, hitting .294/.390/.588 against lefties this season (big leagues and Triple-A combined), will give Brown a natural platoon partner at first.
Charles, 25, is a hard-throwing righty who has battled command issues throughout his time in the minors and struggled considerably this season in his second stint at the Double-A level. He’s yielded 36 earned runs in just 32 innings, thanks to a through-the-roof 19.4% walk rate and a career-worst 1.13 HR/9 mark. Charles can run his fastball into the upper 90s and has had multiple seasons where he’s fanned more than 30% of his opponents, but this year’s 19.4% walk rate is right in line with the career 19.3% mark he’s posted across parts of six minor league seasons. The A’s can put Charles on waivers at any point in the next week.