Undefeated prospect Phil Davis (8-0) has agreed to replace Ortiz. The fight will serve as the main event for the UFC Fight Night 24 card next month in Seattle.
UFC president Dana White broke the news Saturday via his Twitter account.
"Tito has pulled out of fight night due to getting stitches," wrote White. "Phil Davis will be fighting Lil Nog now."
Minutes after White posted the news, Ortiz added a short blurb on his Twitter account.
"[Twenty-two] stitches and a bad concussion," Ortiz wrote. "I will heal and be better soon."
It's currently unknown how long Ortiz (15-8-1) will remain sidelined. The former UFC light heavyweight champion is 0-4-1 in his last five fights and hasn't won since posting a TKO win over Ken Shamrock in October 2006.
White had already described the fight against Nogueira (19-4) as a must-win for Ortiz to remain on the UFC roster. Ortiz's loss, however, is Davis' blessing.
"It's an honor to be able to have a fight made with a legend like him," Davis told ESPN.com. "It's truly an honor."
Davis, 26, is 4-0 since joining the UFC in 2010. This will be his first time fighting in the main event.
Ortiz's injury announcement is the latest wrinkle in a string of changes to the 205-pound division's schedule.
Davis had originally been expected to fight Matt Hamill at UFC 129 in April. That fight fell through when it was announced Hamill was replacing Thiago Silva in a fight against Quinton Jackson at UFC 130 in May. Jason Brilz ended up taking Hamill's place against Davis, however, with Davis now gone, he's the one without an opponent for UFC 129.
UFC Fight Night 24 will take place at Key Arena in Seattle. In addition to the main event, the card features a welterweight bout between Dan Hardy and Anthony Johnson, as well as a rematch of a controversial fight between Nam Phan and Leonard Garcia.
Brett Okamoto covers mixed martial arts for ESPN.com. Franklin McNeil contributed to this report.