According to mmaweekly.com:
The news of Silva’s removal from the card was confirmed to MMAWeekly.com by sources close to the situation on Tuesday.
The nature of the injury to Silva was not disclosed, but the American Top Team fighter has had back issues in the past that have hindered his training.
[UPDATE] – A search for a replacement opponent against Tim Boetsch is underway.
[UPDATE #2] – MMA Weekly reports that the UFC has signed Todd “Bulldog” Brown (11-1) to face Boetsch.