Ertz will undergo an MRI on Monday to confirm the extent of his injury, though the team believes his ACL is still intact.
Ertz made a 12-yard catch late in the first quarter at SoFi Stadium on Sunday, but had his left knee rolled up on as he was tackled. The 32-year-old limped off the field on his own before he was carted off from the sideline.
He didn’t return to the game and was spotted postgame wearing a knee brace.
Ertz has 394 receiving yards and four touchdowns on 68 catches this season, his first full one in Arizona after he landed there midway through last season.
The Cardinals, despite not having Kyler Murray, picked up a 10-point win Sunday behind a 238-yard day from backup quarterback Colt McCoy. The Rams started John Wolford instead of Matthew Stafford due to a concussion, and they lost star receiver Cooper Kupp with a leg injury, too.
Arizona will host San Francisco next Monday night. It’s unclear how long Ertz will be sidelined.