According to Albert Breer with the NFL Network, citing a source "briefed on the injury", rookie receiver Mohamed Sanu will be out for two to three months with a stress fracture that he suffered during practice this week.
Geoff Hobson with Bengals.com related the injury to similar injuries by Bengals players, writing that it's:
...an injury similar to the one former cornerback Johnathan Joseph suffered in the 2007 mandatory minicamp and current right tackle Andre Smith got during a 2009 practice. It's the long bone on the outside of the foot that connects to the little toe and it's seen as an injury that Sanu is going to have no problem overcoming. But since it happened with just five weeks left in the season he's got no chance of getting back in time.