The Cardinals said an MRI on Tuesday determined the extent of the injury but showed no more significant damage.
"He had an MRI today that revealed a bone contusion and a right knee sprain," the Cardinals said in a statement on Tuesday. "However, the knee is stable and and there isn't any more significant damage. He'll be evaluated as the week progresses."
Fitzgerald was hurt on the third play of the third quarter of Monday night's game at San Francisco when the 49ers' Dashon Goldson landed on his leg on a deep pass attempt. Fitzgerald was helped off the field and then went to the locker room. He returned to the game with 3:23 to play.
Fitzgerald said on Twitter on Monday night that he was "sore but OK." Asked again on Tuesday, he answered he was "sore but straight."
Arizona coach Ken Whisenhunt said cornerback Matt Ware did not have a torn ACL as was initially feared. Whisenhunt said it is uncertain how much damage Ware had in the knee and when he might return.
"We're fairly certain that it's not an injury that requires an operation, which is a good thing," the coach said.
Kicker Neil Rackers aggravated a nagging groin injury. Whisenhunt said the cool, damp conditions at Candlestick Park were a factor in Rackers' problems.
"We're obviously going to have to assess him and see where it is and if he can continue in this cycle or how we go from there," Whisenhunt said.
If a new kicker is needed for Sunday's game at Detroit, the addition wouldn't have to be made until later in the week, Whisenhunt said.
"You want him to be able to get some reps with the holder, with Ben [Graham]," Whisenhunt said. "It's not as critical as another position where you have to learn a game plan and everything that's involved with that."
Information from The Associated Press was used in this report.