Ian Rapoport of NFL Network reported on Friday morning that Pettis suffered a sprained knee and that it is not considered to be a long-term injury.
Head coach Kyle Shanahan confirmed Rapoport's report on Friday, telling reporters that the second-year receiver will not play on Sunday against the Baltimore Ravens and is week-to-week with a sprained knee.
"He sprained his knee Wednesday," Shanahan said. "Just a non-contact drill. Him and a couple of players got tangled up, and just sprained his knee. He'll be week-to-week with that."
Shanahan wasn't able to clarify on the severity of the sprain.
The 49ers made Pettis a second-round draft pick out of Washington in 2018. He has just 11 receptions for 109 yards with two touchdowns through 11 games this season, but has been held without a catch during San Francisco's last three games.