Cardinals' 2023 Offseason Mariners Swap Now Looks Like a Steal

The St. Louis Cardinals reliever looks like a massive steal for the organization.

May 2, 2026; St. Louis, Missouri, USA; St. Louis Cardinals relief pitcher Riley O'Brien (61) delivers a pitch against the Los Angeles Dodgers in the ninth inning at Busch Stadium. Mandatory Credit: Joe Puetz-Imagn Images | Joe Puetz-Imagn Images

There have been a lot of positive takeaways from the 2026 St. Louis Cardinals so far this season.

Over the course of the campaign so far, much has been said about the young core and the various breakouts happening throughout the roster. 23-year-old outfielder Jordan Walker is the club's biggest bright spot so far this season. He has been incredible and now firmly looks like a long-term core piece. The same can be said about rookie JJ Wetherholt, starter Michael McGreevy, and shortstop Masyn Winn, among others.

Another guy who has gotten a lot of love so far this season has been closer Riley O'Brien. He is 31 years old, which doesn't fit the age of the other guys breaking out throughout the roster. But he has had a bit of an unorthodox run to get here. He made his big league debut as a member of the Cincinnati Reds back in 2021, but he just appeared in just one game. His professional career kicked off back in 2017 after being selected in the eighth round of the MLB Draft by the Tampa Bay Rays. He was traded to the Reds in 2020 and made his big league debut in 2021.

The Cardinals-Mariners Deal Looks Really Good For St. Louis

May 10, 2026; San Diego, California, USA; St. Louis Cardinals relief pitcher Riley O'Brien (61) delivers during the ninth inning against the San Diego Padres at Petco Park. Mandatory Credit: Denis Poroy-Imagn Images | Denis Poroy-Imagn Images

He pitched in one game in the majors in 2021 and one game in 2022 in the majors with the Seattle Mariners. O'Brien spent the entire 2023 season down in the minors before he was traded to the Cardinals for cash. The deal was finalized on Nov. 6, 2023 and now a few years later, it looks like an absolute steal.

O'Brien made just eight appearances in the majors in 2024 due to injuries. His eight appearances in the majors at the time were significantly more than he had with any other team still. In 2025, he was finally healthy and given a legit opportunity and he responded with a 2.06 ERA in 42 appearances. In 2026, he has been even better. O'Brien has a 2.84 ERA in 19 outings to go along with 11 saves. Plus, his advanced metrics are incredible.

You can never truly judge a trade right away. It takes time for the parts to really sink in. If you had judged the O'Brien deal right away, it looked like a minor league dart throw that wouldn't matter in the majors. Now, a few years later, the Cardinals look like geniuses for being able to get O'Brien for cash. Plus, it doesn't hurt that he's under team control through the 2030 season.

There are few deals over the last few years better than the O'Brien deal now.