Bonaddio explains why here. His expectations for Austin in 2013: somewhere around 59 receptions for 961 yards with eight touchdowns.
The Rams would presumably be OK with those types of numbers. However, I think Austin has a chance to exceed that total for receptions while heading to a team with relatively unestablished players at wide receiver.
The chart below ranks rookies since 2002 by most receiving yards while including their stats for receptions and receiving touchdowns. The projections for Austin would put him in the top 10 by that standard.
The Rams haven't had a receiver with 961-plus yards since Torry Holt had 1,189 yards in 2007.