Crabtree tore his Achilles and underwent surgery to repair the injury back in May.
It was believed then that the breakout 2012 star -- Crabtree absolutely exploded once Colin Kaepernick took over at quarterback midway through the season -- would help turn the Niners into a much more explosive offense in 2013, but the injury derailed all that before it began.
Since May Crabtree's been reportedly targeting to miss the first 10 weeks or so of the season at minimum. (A return in Week 12 has been optimistically thrown around; Terrell Suggs returned for the Ravens from an Achilles injury by Week 7, setting the stage for expectations to get too high.)
Putting him on the PUP list, which means he can't practice or play through the 49ers first six games of the year, locks down the timeline a little bit in that it will definitely keep Crabtree out through the first six games of the year.
Crabtree's injury put a major void in the 49ers wide receiver corps: Anquan Boldin is the top target for Kaepernick with maybe Quinton Patton, a rookie out of Louisiana Tech, closing in to help out. Vernon Davis will be a big boon for Kaep until Crabtree's able to return (we assume this will happen but it's no guarantee) but it's clear that the team will miss its top wideout through the first few weeks of the season at minimum.