Coach Mike Tomlin confirmed the news in his postgame interview and didn't provide any additional information.
Conner, who appeared briefly in the locker room but did not do interviews, was hurt on a tackle late in the fourth quarter. Conner gained 10 yards on a screen pass with 5:08 remaining and fell to the grass after pushing through two Chargers defenders for the first down. Rookie running back Jaylen Samuels finished the drive with a 10-yard touchdown catch.
Quarterback Ben Roethlisberger said he didn't want to speculate about Conner's injury.
After a rough two-week stretch that included a crucial fumble in Denver, Conner got going Sunday with 60 rushing yards and two rushing touchdowns on 15 carries. He added 14 receiving yards on three catches.
Conner is among the league leaders in rushing touchdowns, with 12.
"You never want to see a guy like that go down," said Steelers wide receiver Antonio Brown, who recorded a season-high 154 yards on Sunday. "But like we said, every guy in a helmet is upholding the standard, and anyone can be the reason we win."