That could mean the four-time Pro Bowl receiver won't play again this preseason.
Fitzgerald watched practice on Monday in a leg brace. Afterward, he called the injury "a minor setback" but wouldn't set any time for his return.
"I don't set timetables," he said. "I'm just going to do everything they ask me to do, plus some, and hopefully that's going to get me back as fast as possible."
Coach Ken Whisenhunt said that while the team will take a conservative approach, he fully expects Fitzgerald to be back for the season opener at St. Louis on Sept. 12.
Fitzgerald sprained his MCL when he was hit in his only reception Saturday night in a 19-16 preseason win over Houston. He caught the 16-yard pass from Matt Leinart over the middle but took a nasty hit from safety Eugene Wilson. Fitzgerald stayed in for another series before leaving the game for good.
He said any lost time in training camp is significant.
"I'm playing a position that's predicated on timing," Fitzgerald said. "You know we have a new quarterback. I need to be out there with him every day, not just on Sunday. I need to be out there Monday through Saturday working with him every single day to build our continuity. So any time you miss as a receiver is not good."
Fitzgerald enters his seventh NFL season widely recognized as one of the best receivers in the game with 523 career receptions for 7,067 yards and 59 touchdowns. Last year, at 26, he surpassed Randy Moss as the youngest player in league history to top 7,000 yards receiving. In six postseason games, Fitzgerald has 42 catches for 705 yards and nine scores. He has topped 1,000 yards receiving each of the last three seasons and four times in his career.
He has been remarkably durable.
Counting the playoffs, Fitzgerald has played in 47 consecutive games and has missed only four -- out of 102 -- in his career.
Fitzgerald acknowledged this sprain is worse than the one that knocked him out of the team's loss at San Francisco last Dec. 14. He came back six days later to catch a touchdown pass in a victory at Detroit that clinched the Cardinals' second straight NFC West title.
Fitzgerald worked out regularly in the summer with Leinart, who has replaced the retired Kurt Warner, and had said he was having the best training camp of his career.
"That's the thing that's most frustrating," he said. "You put in so much time in the offseason. You work so hard to get your body in shape, then something like this happens and you kind of take a couple of steps back. It really makes you count your blessings and think about how fortunate you are."
With Fitzgerald's longtime receiving mate Anquan Boldin traded to Baltimore, Steve Breaston will fill the No. 1 receiver spot. Until Fitzgerald returns, Early Doucet moves up to No. 2. Doucet sat out Monday's practice to rest sore abdominal muscles.