Roster sizes in Major League Baseball shrank Monday, and the Mets made the most notable cut, parting ways with second baseman Robinson Canó, who has struggled in the twilight of his career.

Canó, 39, may not be a free agent until next week because he was designated for assignment, but the Mets are moving on from a player who had been a centerpiece of their blockbuster 2018 trade with the Seattle Mariners. In removing Canó from their 40-man roster, the Mets, who entered Monday with the best record in the National League, are prepared to eat the remaining portion of the $40.5 million they owed him entering this season.

Returning from his second performance-enhancing drug suspension, a yearlong ban that cost him the entire 2021 season, Canó was hitting .195 with a .501 on-base plus slugging percentage in 12 games this year. He was well liked in the Mets’ clubhouse, but in 43 plate appearances, he had two walks and one extra-base hit.

An eight-time All-Star and a 2009 World Series winner with the Yankees, Canó had the makings of a Hall of Fame résumé before sputtering over the last several seasons with his legacy tainted by the suspensions.

Because the labor dispute in baseball cut the length of 2022 spring training in half, M.L.B. and the players’ union agreed to expand rosters to 28 for the first month of the season in hopes of preventing injuries. On Monday, the roster sizes returned to the usual 26, but with an extension through May 29 of a 14-pitcher cap. Instead of demoting a more capable younger player to the minor leagues, such as Dominic Smith, a 26-year-old first baseman, the Mets made the more painful financial choice.

When the Mets traded for Canó before the 2019 season, his former agent, Brodie Van Wagenen, was the Mets’ general manager. He and the Wilpons, who owned the Mets, believed Canó could perform into his late 30s. Canó was coming off an injury and his first P.E.D. suspension (80 games).

To swing the trade for Canó and closer Edwin Díaz, the Mets mortgaged part of their future by giving up three prospects — including two first-round draft picks, outfielder Jarred Kelenic and pitcher Justin Dunn — and absorbed $100 million of the 10-year, $240 million free-agent contract that Canó had signed with the Mariners.

Canó will be paid $48 million in 2022 and ’23, regardless of whether he plays, with the Mariners contributing $7.5 million toward those seasons. But under Steven A. Cohen, the Mets’ second-year owner, the team has behaved as if money is not an issue in its pursuit of a championship. The Mets’ payroll of $288 million, for luxury tax purposes, trailed only the Los Angeles Dodgers in M.L.B., according to Cot’s Baseball Contracts.

The December 2018 trade that brought Canó to the Mets was questioned by many at the time, and has played out in surprising ways. Kelenic has sputtered to start his major league career with the Mariners, while Dunn, now injured, turned into a useful pitcher before his March trade to the Cincinnati Reds. Díaz has been the best of the players so far, establishing himself as a solid closer after a rocky first season in Queens. On Friday, he was dominant in one inning while finishing out the second no-hitter in Mets history.

As for Canó, he battled injuries over his first two seasons with the Mets but hit .275 with 23 home runs in 156 games. Then came the second P.E.D. suspension — and no explanation for how he tested positive again — and the slowed bat in 2022. And with that, the Mets said goodbye.