Per Okamoto, the two have yet to officially sign contracts but made the necessary commitments to line up a matchup inside the Octagon.
Pettis is coming off a unanimous decision over Donald Cerrone at UFC 249 in May. He's the No. 12 welterweight in UFC's official rankings.
"I'm not obsessing about getting back to a belt, or getting that next spot," the 33-year-old said to Okamoto. "This is my first chance to get back-to-back wins in a while. Let's start back over, take this one step at a time. I feel amazing at 170 pounds. This opportunity opened up and I couldn't say no to it."
Morono will be looking to make a statement. He defeated Rhys McKee by unanimous decision at UFC Fight Night 182 in November. Beating a former lightweight champion—a little more than one month after his last fight—would represent the biggest win of his career.
The Dec. 19 Fight Night card will feature 14 fights between the prelims and main show. Leon Edwards vs. Khamzat Chimaev is still listed as the featured bout, but MMA Fighting's Damon Martin reported that honor will likely fall to Stephen Thompson and Geoff Neal.