Mendes was expected to meet Clay Guida, but an injury knocked Guida out of the featherweight contest.
Elkins (16-2) recently improved to 5-0 at 145 pounds after defeating Antonio Carvalho.
Mendes (13-1) is a former top contender to the UFC title, as his loss to Jose Aldo is the only one of his career.
The bout is expected to be a featured one on the FX-televised prelims April 20.
Well, Elkins stated his desire to face a Top-10 ranked featherweight after defeating Carvalho, but I’m not sure he meant someone like Mendes right away. This will be the biggest test of his career, but a win would vault him right into contender status.