Mills was acquired by the Red Sox from the Royals in exchange for Jacob Wallace on Friday, Chris Cotillo of The Springfield Republican reports.
Mills appeared in 27 games between the Mariners and Royals in 2022 and had a 4.60 ERA, 1.33 WHIP and 26:13 K:BB across 29.1 innings. The 27-year-old should see the majors at some point in 2023 even if he doesn't make the Opening Day roster, but he'll likely fill a low-leverage role.