Favre hasn't had an official diagnosis, but he says taking a deep breath causes discomfort. The 39-year-old quarterback wasn't listed on the injury report and no other other Vikings official mentioned the injury pregame.
The three-time MVP says he has no plans to wear any kinds of special padding, saying "the damage is done." Favre started and is expected to play the entire first half against the Houston Texans.
Favre didn't appear to be in any pain when he handed off to Adrian Peterson on the first play of the game, and Peterson scored on a 75-yard run.
Favre's agent Bus Cook wrote in an e-mail to The Associated Press that he didn't know if Favre has a cracked rib.
Favre, who signed a $25 million deal with the Vikings on Aug. 18, completed just one pass for four yards over two series in his Minnesota debut just days later.
Information from The Associated Press contributed to this report.