Ramirez left after throwing two innings in his most recent start, the Angels' 7-3 defeat to the Oakland Athletics last Saturday.
The 2018 season is a little over a week old, but Ramirez is already the fourth Angels pitcher to hit the disabled list, and he's the second who could potentially be out of action until 2019. Alex Meyer had shoulder surgery last September that will likely keep him out for all of this year.
Ramirez had a relatively successful transition to the starting rotation after working as a reliever for the entirety of his MLB career prior to 2017. He finished 11-10 with a 4.15 ERA and a 4.71 FIP in 27 appearances, according to Baseball Reference.
While Los Angeles wasn't expecting Ramirez to be the team's ace in 2018, he was set to be a nice option at the back of the rotation. His absence puts further strain on what was already a lack of pitching depth for the Angels.