It's a minor-league deal, Joel Sherman of the New York Post reported.
Rosenthal was designated for assignment by the Detroit Tigers two weeks ago and elected to become a free agent on Aug. 11. He had a 7.00 ERA in 10 appearances for Detroit.
The 29-year-old right-hander began 2019 with the Washington Nationals, with whom he logged a 22.74 ERA in 12 appearances, including 15 walks in 6 1/3 innings.
Rosenthal can still light up the radar gun, as he's averaged 98 mph on his fastball in 2019, but finding the strike zone has been a different story. He combined to walk 15.3 hitters per nine with the Tigers and Nationals this season.
Before undergoing Tommy John surgery in 2017, Rosenthal was a dominating reliever for the St. Louis Cardinals, saving 121 games for the team overall, including 48 in 2015.