Richards is scheduled to have surgery next week and has been placed on 60-day disabled list by the Angels.
Richards was injured in the second inning of the Angels' 8-3 win over the Boston Red Sox on Wednesday night. Boston had runners on first and second when Red Sox third baseman Brock Holt hit a grounder to Angels first baseman Albert Pujols.
Richards went to cover first base, but he never made it, falling to the ground and holding his left knee. Richards was taken off the field in a stretcher after being seen by trainers.
Yahoo Sports' Jeff Passan reports that Richards tore his patellar tendon in the "best spot possible," and he should be ready for spring training.
Richards is 13-4 with a 2.61 ERA this season for the Angels, who hold a 1.5 game lead over the Oakland Athletics in the American League West.
- Scooby Axson and Sarah Barshop