The Angels were 19-7 (.731) in games Garrett Richards started this season.

Los Angeles Angels pitcher Garrett Richards is expected to miss 6-9 months after an MRI exam Thursday revealed a torn patellar tendon in his left knee. He sustained the injury while running to cover first base in Wednesday’s start against the Boston Red Sox.

With Richards done for the year, let’s review his breakout season and his importance to the Angels.

Breakout season

Richards entered the season 9-9 with a 4.37 ERA in 29 career starts (167 innings), but the 26-year-old figured things out this year. Richards will finish the year 13-4 with a 2.61 ERA over 168 2/3 innings. In 1 2/3 more innings this year, he gave up 32 fewer runs as a starter than the previous three years.

He has become one of the best starters in the American League, ranking first in opponent slugging percentage (.261), second in opponent batting average (.201), eighth in ERA (.261), 10th in strikeout rate (24.2 percent) and sixth in ground-ball rate (51.3 percent).

Richards’ opponent slugging percentage is the fourth lowest in a season in Angels history and the lowest since Nolan Ryan in 1977. It’s also the third lowest by any pitcher in the last 20 seasons. The only two lower? Pedro Martinez (.259) in 2000 and Greg Maddux (.258) in 1995, both of whom went on to win Cy Young awards those years.

So just how did Richards put it together this season? His plus stuff finally started missing bats. From 2011 to '13, Richards’ strikeout rate was only 15.2 percent as a starter. That ranked 168th out of 230 pitchers who started at least 20 games over that span. This season, he has increased his strikeout rate to 24.2 percent, which ranks 16th out of 95 qualified starters. In fact, only one starter in baseball has increased his strikeout rate more than Richards from last season to this season (Brandon McCarthy 8.0 percentage points; Richards 7.9).

Importance to Angels

The Angels are 19-7 (.731) when Richards starts this season and 57-43 (.570) when anyone else does. According to Elias, the Angels' .731 win percentage with Richards on the mound is the best for any starter in the American League this season (minimum 15 starts).

Angels Starting Pitchers This Season

Richards has been worth 4.4 Wins Above Replacement this season. Entering Thursday, all other Angels starters had been worth 3.3 WAR combined. The loss of Richards could have an effect on the Angels' bullpen as well. No bullpen has thrown more innings since the All-Star break than the Angels (122).

Replacing Richards

While it’s unclear who will take Richards’ spot in the rotation the rest of the season, the Angels will surely look for their other starters to step up, as Matt Shoemaker did Thursday night against the Red Sox.

Shoemaker no-hit the Red Sox through 6 2/3 innings and finished with 7 2/3 scoreless innings along with nine strikeouts. Since giving up a career-high eight runs against the Kansas City Royals on June 27, Shoemaker is 5-2 with a 2.51 ERA in seven starts.