Nelson rode out his UFC contract earlier this year, but then quickly inked a deal with Bellator in May. He will make his debut with the company against Javy Ayala (10-5) on Sept. 23 at the SAP Center in San Jose, Calif.
Henderson (24-7) will make his return to the cage against Patricky Freire (17-8).
While Freire enters the fight coming off of a victory over former Strikeforce champion Josh Thomson, Henderson will not have fought in nearly a year when the two finally collide.
Henderson fell short in his bid to take the lightweight belt from Michael Chandler in November. He then had to undergo surgery to repair a torn ACL.