1) James McCann
After being nothing more than a light-hitting catcher with the Tigers, James McCann turned into one of the best catchers in the American League with the White Sox. He was an all-star in 2019 and had an OPS over 100 points higher in the shortened 2020 season. His bat and glove had improved dramatically with the White Sox making him an appealing free agency target.
At the time McCann was developing into a strong catcher, the best one in all of baseball without a doubt was J.T. Realmuto who fittingly was also a free agent at the end of the 2020 season. The Mets with new owner Steve Cohen had a chance to steal Realmuto away from their division rivals, but did not. They opted to sign McCann before Realmuto agreed to his deal.
The terms of the deal based on the fact that McCann had only been good for a year and a half felt a bit extreme, but Mets fans entered this new beginning with an open mind. After getting no production from catchers in years, it was nice having a guy who could seemingly both hit and defend.
Immediately after signing his four-year deal worth $40 million to join the Mets, McCann turned back to the hitter he was in Detroit. He was one of few Mets able to stay healthy for most of the 2021 season, but had just a .643 OPS that season. He wound up performing even worse offensively in 2022, posting a .538 OPS and losing his starting job to fellow light-hitting catcher Tomas Nido.
In order to get rid of McCann, the Mets had to eat most of his contract and take on nothing in return. The Mets didn't need McCann to be an all-star, but expected him to be a capable hitter and solid defender. The defense was mostly there, but the bat was abysmal while Realmuto continued to be among the best in the game at the position in Philadelphia.