[6] On August 10, 2011, Richards was called up to take the rotation spot of Joel Piñeiro, filling in for the suspended Jered Weaver.

[7] His major league debut did not go well, as Richards took the loss to the New York Yankees, giving up 6 earned runs in 5 innings pitched.

In 10 starts during 2012 for the Angels Triple-A affiliate, the Salt Lake Bees, Richards was 5–2 with a 4.31 ERA, striking out 48 while walking 29.

He was called up by the Angels on June 5, 2012 to replace Jered Weaver who was placed in the 15-day disabled list[8] and won his first major league game against the Seattle Mariners, pitching seven strong innings and allowing only one earned run.

[10] On August 20, 2014, Richards suffered a knee injury while attempting to complete a double play at first base at Fenway Park during a game against the Boston Red Sox.

[11] Richards was diagnosed with a torn patellar tendon in his left knee and it prematurely ended his 2014 season.

[13] The injury required surgery, meaning that Richards would take no further part in baseball activities for the remainder of the 2016 season.

[19] On February 3, 2021, Richards officially signed a one-year, $10 million contract with the Boston Red Sox.

[20] He made his Red Sox debut on April 4, taking the loss in a start against the Baltimore Orioles after allowing six runs on seven hits in two innings.

[21] On June 16, Richards notched his first MLB career hit, an RBI double off of Atlanta Braves starter Ian Anderson.

[24] This move occurred days before the return of starter Chris Sale, who made his first appearance of the season on August 14.

[26] Overall during the regular season, Richards made 40 appearances (22 starts) while compiling a 7–8 record with 4.87 ERA and three saves; he struck out 115 batters in 136+2⁄3 innings.

[19] On November 7, the team declined to exercise their $10 million option on Richards for 2022, making him a free agent.

Joe Girardi, known for serving as manager of the New York Yankees, noted that Richards' fastball is exceptional because "its movement... is not consistent".