
Heavy MMA first reported the news on Tuesday afternoon, and MMA Fighting has confirmed it with sources.
Aldo (18-1), the final WEC featherweight champion, most recently defeated Manny Gamburyan in September. He captured the 145-pound title in November 2009 when he defeated Mike Brown via second round TKO.
Grispi (14-1) was supposed to fight Erik Koch at WEC 52 earlier this month, but was pulled from that fight card following the announcement of the UFC-WEC merger in October.
No word on which bout will replace Aldo-Grispi as the UFC 125 co-main event.
UFC 125 is scheduled for Jan. 1 in Las Vegas. The event, which will air live on pay-per-view, will be headlined by Frankie Edgar vs. Gray Maynard for the UFC lightweight title.