He will undergo an X-ray and an MRI on Monday in San Diego.
Gates injured his left ankle in the second quarter. He came back on the field for a series before he was taken to the locker room late in the first half. He was not on the sideline for the second half.
Gates got hurt blocking on a carry by running back Ryan Mathews.
"I got tripped up and I guess some guys fell on the back of my leg," said Gates, who was wearing a walking cast in the locker room. "That's initially how it started. Then on a pass down the middle on the last play I was in, someone fell on my ankle. I just couldn't go. I couldn't move."
San Diego coach Norv Turner said he is unaware of how seriously Gates is hurt.
"Antonio couldn't come back. I don't know the extent of it (his injury). I know it was serious enough that he couldn't return," Turner said. "Hopefully, it's not a long-term deal."
Gates, a six-time Pro Bowl selection, finished with two catches for 12 yards. He had scored in nine consecutive games, an NFL record for tight ends.
Losing Gates did not change the Chargers' offense too much, Rams free safety Oshiomogho Atgowe said.
"They kept their offense pretty much the same. What happened was they lost one of their prime weapons in Gates," Atgowe said. "We knew their offense wasn't going to drop off all that much even though they lost the caliber of Gates.
"Philip Rivers is a great quarterback. He makes plays and he knows how to get the ball to his playmakers. We knew there would be no great drop-off without Gates. We just had to keep playing regardless of who was out there. We did a good job."
Entering Sunday's game, Gates had 29 receptions for 478 yards and an NFL-leading seven touchdowns.