The Cardinals acquired Rivera from the Red Sox on Tuesday in exchange for Juan Garcia, Andrew Parker of SoxProspects.com reports.
Rivera had been designated for assignment Monday as the Red Sox did some 40-man roster housekeeping. The big lefty allowed seven runs over 10 innings during his brief time in the majors in 2026, and he posted a 2.96 ERA and 55:27 K:BB across 45.2 frames with Triple-A Worcester. Rivera will compete for a spot in the Cardinals' bullpen in 2027, but he has minor-league options remaining.