"You're going to be thinking of Garrett once every fifth day when he was out there," manager Mike Scioscia said. "You might not be able to replace that, but it doesn't mean that we're not going to be able to reach our goal as a cumulative pitching staff."
First baseman Albert Pujols had fielded a ground ball by Brock Holt and thrown to second for a force play with Boston runners on first and second. Richards never made it to first base, as he fell to the ground, clutching his left knee in pain.
General manager Jerry Dipoto said Thursday "we'll be open" about finding a deal to replace Richards in the rotation. The Angels selected the contract of left-hander Wade LeBlanc, who pitched briefly for the New York Yankees earlier this season, from Triple-A Salt Lake.
Dipoto said that nine months is an absolute worst-case scenario, adding that the injury "should not affect" Richards' 2015 season.
Richards, who was taken off the field on a stretcher, flew back to Los Angeles on Thursday to be examined by team doctors.
Center fielder Mike Trout called it a tough loss but said the team has to keep winning.
"We're going to take it one game at a time," Trout said. "We can't get ahead of ourselves. It's tough losing Garrett, for sure. But we just have to keep winning ballgames. That's all that matters."
"I think it's really uplifting," Scioscia said afterward. "I think it's important. There's going to be a lot of attention on our rotation obviously with Garrett not going to be part of it. You can't say enough about what Matty did tonight."
Richards took a 13-4 record and a 2.53 ERA into Wednesday's game in what has been a breakout season for the 26-year-old right-hander. He entered the game having struck out 100 batters in 100 1/3 innings since June 4 while posting a 1.79 ERA. Richards was tied with Felix Hernandez for the American League lead in batting average against.
This is another blow to the Angels' rotation. Tyler Skaggs has already been lost to Tommy John surgery that will sideline him until the start of the 2016 season.
On Monday, the Angels took over sole possession of first place in the AL West for the first time since May 15, 2011.
Information from The Associated Press was used in this report.