According to MMAWeekly:

UFC fighter Rousimar Palhares suffered a fractured tibia in training while wrestling. He underwent surgery on Thursday night at Hospital Miguel Couto in Rio de Janeiro, Brazil, according to a report by Martins Denis on intheguard.tv.

MMAWeekly.com independently confirmed the injury, which will keep the Brazilian Top Team fighter out of a planned bout against American Top Team fighter Alessio Sakara at UFC 101.

I was really looking forward to this fight, as I'm a fan of Palhares.  I think he had a very good chance of winning his bout against Alessio Sakara.  There's no word on who will replace Palhares on the card at this time.  There are ten rumored fights for UFC 101, so Sakara could very well be moved to another card once a replacement is found for Palhares.