At 5:03 of the second, Backstrom absorbed a heavy check from Kings defenseman Drew Doughty near the Washington bench with his left shoulder and took the brunt of the collision. Backstrom's shift ended four seconds later and he did not return.
Washington lost 5-4 in a shootout.
The 26-year-old is one of Washington's offensive catalysts; he has 13 goals and 70 points in 73 games this season.
Without Backstrom, the Capitals blew a 3-1 third-period lead and ended up losing in the shootout. Oates, however, did not use that as an excuse.
"Obviously you miss [Backstrom], but every team has guys that go out," Oates said. "You've got 19 other guys that have got to do the job. We have plenty of guys who can do the job."