Richards was hurt in the second inning while running to cover first base on a potential double-play grounder by Boston's Brock Holt. Richards' fell in the dirt near the base. Several minutes later he was wheeled off while he raised his right arm to the crowd and put his left hand on his forehead.
The Angels said he sustained a patellar injury but didn't disclose the extent.
With runners at first and second, Holt grounded to first baseman Albert Pujols, who threw to shortstop Erick Aybar for the out at second. Richards then fell while running to first for a return throw that never came.
Aybar spun and threw to third, but Xander Bogaerts, who had run past the base, dove back safely.
Cory Rasmus replaced Richards.
Richards (13-7) was 7-2 in his previous 11 starts, lowering his ERA from 2.87 to 2.53. Since June 4, he had a 1.79 ERA with 100 strikeouts in 100 1-3 innings.
But he struggled on Wednesday, allowing two runs and five hits with one walk and no strikeouts in 1 2-3 innings.