In 2018, Carson Kelly looked like he would be the Cardinals' catcher of the future. Ranked their 3rd best prospect and the 46th-best prospect in baseball, Kelly looked like he would be a plus defender with an above-average bat at the catcher position. Instead, he was shipped to the Arizona Diamondbacks in the Paul Goldschmidt trade and hasn't lived up to expectations for them.
For his career, Kelly is slashing .223/.309/.383 with 43 HR and 157 RBI in 415 games. He's been a fine catcher in his career but hasn't come close to the ceiling he once had. He was relegated to backup catcher duties this year with the acquisition of Gabriel Moreno, and may not be anything more than a below-average starter at this point.
It's safe to say the Cardinals are pretty happy with their decision to part with Kelly in the Goldschmidt trade. Who knows if the Cardinals' felt like Kelly was being overvalued by the rest of baseball, they turned him into the 2022 NL MVP, which is a huge win.