The 29-year-old from Cole Harbour, N.S., knocked out veteran Gray (The Bully) Maynard at UFC 160 in May to earn a title shot against Benson (Smooth) Henderson.
But that opportunity went instead to former WEC champion Anthony (Showtime) Pettis when a concussion sustained in training sidelined the Canadian. Pettis made the most of his chance, submitting Henderson in August to win the 155-pound crown.
The UFC said Grant would get first crack at Pettis, with a televised card Dec. 14 in Sacramento the likely stage.
Grant, however, explained in a series of tweets that he is still injured.
"Unfortunately I won't be fighting Pettis on Dec. 14," he said. "I am still not yet 100% (and) can't commit to fighting. 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's I'm gonna do. I'm not bitter."
Added Grant: "I accept bad things when I step into (the) cage. But long term health is #1."
Josh Thomson (20-5) is the lucky replacement this time. Thomson, ranked fifth among lightweight contenders, will take on Pettis in the main event at Sleep Train Arena.
Grant (21-5) is 5-0 since dropping down to lightweight. He is currently the No. 3 contenders behind former Strikeforce champion Gilbert Melendez and No. 1 Henderson.