1) INF Edmundo Sosa for RP JoJo Romero (2022)
Leading up to the 2022 trade deadline, the Cardinals were rather quiet. They waited until July 30th to make their first deal, and it's safe to say that their opening move left many fans unsatisfied. The Cardinals announced that they'd acquired left-handed reliever JoJo Romero in exchange for infielder Edmundo Sosa.
Sosa was coming off of an excellent 2021 season. Filling in for several injured players, Sosa was an above-average producer at the plate, and a surprisingly effective defender. Though he made several critical errors, including a throw that nailed an umpire's head, he was flexible and played multiple positions at a high level. Unfortunately, 2022 wasn't going his way. Sosa had regressed significantly at the plate, and with the return of Paul DeJong, Sosa had no discernible role.
Thus, the Cardinals traded him for Romero, an unproven reliever best known for crushing (no, literally crushing them) cans of Red Bull before appearances.
Since that trade, Sosa has been solid enough as a Phillie. He's just below league average in terms of offensive production, and while he may be better than Taylor Motter or Jose Fermín, it's unlikely he'd have been able to carve out any sort of role in St. Louis. Between Gorman, Winn, Edman, Donovan, and previously DeJong, there just weren't enough at-bats to go around.
Meanwhile, Romero has been the best lefty option out of the bullpen in 2023. He was consistent and trustworthy enough to step into the closer role once Jordan Hicks departed at the 2023 deadline. Romero, who was solid down the stretch in 2022, has been outstanding! His 3.55 ERA is solid, but metrics hint that he's actually been very unlucky. He's running a career-high 10.1 K/9 and has managed to drop his walk rate by over 50%. Romero has turned into a bona fide shutdown reliever. Though this deal got little attention at the time, it's now a clear win for the Cardinals.