Grant, who earned a title shot in May but subsequently found himself sidelined with a concussion, removed his name from the running late Monday night.
"Quick update. Unfortunately I won't be fighting Pettis on Dec 14." Grant tweeted.
"I am still not yet 100% and can't commit to fight. UFC is going with someone else instead. It sucks but it is the best for both myself and the UFC. Please don't worry. I'll be back and if I gotta fight someone else to get back to where I wanna be then that's what I'm gonna do. I'm not bitter."
Pettis' jiu-jitsu coach, Diego Moraes, confirmed to MMAFighting.com's Guilherme Cruz that no opponent has yet been signed for the event. Pettis is still undergoing knee rehabilitation and expects to return to training in mid-October.
The news marks yet another bump in an already rough road for Grant.
Since dropping down from welterweight to lightweight, the 29-year-old racked up a sterling 5-0 record, punctuated by a two-minute starching of Gray Maynard at UFC 160. However lingering concussion symptoms ultimately handed Grant's hard-earned title shot against Benson Henderson to Pettis. And now, once again, he's forced to give up his spot in line.
Grant hopes to climb back into action sometime in 2014, although UFC officials have yet to confirm whether he will receive another title shot upon his return.
"I don't care at this point," Grant told MMAFighting.com's Ariel Helwani. "Just get healthy and see what happens."
Update: Pettis will now face Josh Thomson in UFC on FOX 9's main event.