Got this from the official Bulls press release email list I'm on:
Gray developed lower leg pain over the last two days. He was examined
by team physician Dr. Brian Cole of Midwest Orthopedics at Rush
University Medical Center and it was determined that he has a stress
fracture of his left fibula. The diagnosis was confirmed by MRI. He
is likely out 6-8 weeks.
Gray doesn't seem like he's going to be a big factor for the Bulls anyway, but this is certainly going to hurt his start to the season. On the plus side he won't miss much regular season if he heals quickly, but for a guy like Gray, preseason and training camp might be just as important as the regular season to earn minutes.
Edit to add..
Had to run some errands while the news was breaking, but in my rush to get it up, I didn't really evaluate all the possible consequences of this.
Aaron Gray: As stated above, missing out on training camp/preseason is a big deal for a guy like Gray. He needs to earn playing time in order to get any, and he'll have a much harder time doing that without being able to practice until two weeks into the season. Guys get a chance to prove themselves in preseason, now Gray's best hope for playing time is the failure of his teammates rather than his own success. Not a fun way to root for anyone.
Taj Gibson: Gibson seems to be the most direct beneficiary of this news. He and Gray were likely going to fight over 5th big man minutes, and with Gray out, Gibson will have the chance to prove he belongs as the next guy in line. He still probably won't play a whole lot though.
James Johnson: He might get a slight up tick in playing time. Clearly he's not playing any center, but minutes Gray had may have forced Noah to play some PF. The Bulls will likely cover any minutes he loses by playing a PF more minutes, so Johnson may benefit in a small way.
All in all, I didn't see Gray playing more than 10 minutes a game while only appearing in about half the games. If he misses all of November that might amount to 50-80 minutes or so of total playing time divied out to other players.