This was to be Rosenthal's final year of arbitration eligibility before becoming a free agent. Instead, he enters the pool of available relievers.
• Hot Stove Tracker
"We knew we weren't going to tender him come the tender time, and we decided it made the most sense to do it sooner and let everybody sort of figure out what the options are," general manager Michael Girsch said.
One of those options, which the Cardinals would consider, is bringing Rosenthal back under a different contract structure. That could be through a Minor League deal or with a multiyear contract that runs beyond his expected recovery. Other clubs are expected to have similar discussions with him.
"Obviously, we weren't close to getting something done or else we wouldn't have done this," Girsch said of early exploratory talks. "But if things work out, that'd be great."
Rosenthal, who has been with the organization since being drafted in '09, earned a salary of $6.4 million last season.
After a turbulent 2016 in which he was replaced as closer, Rosenthal reasserted himself as a reliable late-innings reliever before an elbow injury abruptly ended his 2017 season. He had reclaimed the job as closer and had surrendered one earned run in 15 appearances before walking off the mound injured during an Aug. 16 appearance at Fenway Park.
Days later, Rosenthal underwent season-ending surgery.
Had he ended the season healthy, Rosenthal likely would have had been in line to close for the Cardinals next year. The club instead had to pivot in its plans and will spend the offseason fortifying the back end of the bullpen via trade and/or free agency.
Rosenthal, who made his Major League debut in 2012, ranks sixth all-time in Cardinals history with 121 career saves. He set a franchise single-season record with 48 saves in 2015 and is one of six Cardinals to collect at least 10 saves in four consecutive seasons.
In addition to releasing Rosenthal, the Cardinals outrighted infielder Alex Mejia and catcher Alberto Rosario on Monday. Rosario becomes a Minor League free agent. Mejia has been placed on the organization's Triple-A roster.
With the moves, the Cardinals have 35 players on their 40-man roster.