The 26-year-old right-hander went down while covering first base after a ground ball from Brock Holt.
Richards was 13-4 and had an ERA of 2.53 heading into the night with a 3.28 K/BB ratio in 167 innings. His injury comes in the same month that another Angels pitcher, Tyler Skaggs, was ruled out for the year because of Tommy John surgery.
The Angels were 74-50 and first in the AL West heading into Wednesday's matchup in Boston.
- Marc Weinreich