Robertson, 34, was limited to seven games this season after injuring his elbow April 14. He had been healthy throughout his MLB career before arriving in Philadelphia, making at least 60 appearances in the previous nine seasons.
The Phillies signed Robertson to a two-year, $23 million contract in January to serve as their closer. This injury means he'll almost certainly finish his Phillies career with seven total games in uniform. The team has a club option at $12 million for 2021, but it seems unlikely they would exercise it for an aging closer coming off Tommy John surgery.
"Obviously, I've had those thoughts," Robertson told reporters of the possibility his career could be over. "It's tough for me to deal with that."
Robertson spent his first seven MLB seasons with the Yankees before signing with the Chicago White Sox in 2015. The White Sox sent him back to New York midway through the 2017 season, and he spent the next season-and-a-half with the Bronx Bombers.
The Phillies also announced starter Jake Arrieta will have season-ending surgery on his elbow Saturday. The team is currently tied for the second wild-card spot after running off a four-game winning streak, including a three-game sweep of the Chicago Cubs.
Philly was already planning for the rest of this season without Robertson, so this won't affect its outlook. However, the team will almost certainly be back out on the market for a top-flight reliever this winter in hopes of shoring up its ninth inning.