"As the 49ers continued their pursuit of a top-flight cornerback Thursday, concerns emerged about their No. 1 receiver.

Michael Crabtree will be sidelined for up to six weeks of training camp, a source told The Sacramento Bee. The 49ers are worried enough about their receiving corps that they engaged in trade talks for Chad Ochocinco, who instead was shipped from Cincinnati to New England.

The 49ers placed Crabtree on the physically unable to perform list Thursday night, meaning he will not count against the 90-man training camp roster limit until he is activated.

Crabtree sustained a foot injury June 6, the first day of the 49ers' player-organized workouts at San Jose State. Teammates downplayed the injury at the time, saying Crabtree was having a bad reaction to new cleats -- an explanation that drew more than a few snickers.

Tight end Vernon Davis was the only one who hinted at a deeper problem, saying on June 30: "I think there's something seriously wrong with his foot. He'll be ready once camp rolls around. But I think he's staying off of it."

The 49ers have declined to comment on the details of Crabtree's injury. The former first-round pick out of Texas Tech has struggled with foot issues before, having sustained a stress fracture that required surgery in 2009.

Based on the current roster, Josh Morgan emerges as the No. 1 receiver with Ted Ginn playing the opposite side."