MacDonald (21-6-1) says the break has made him a better fighter.
"I got to work on a lot of things and make improvements and get stronger in different areas," he said in an interview. "I felt like the biggest thing was for me to take a mental break from the sport and come back fresh."
Millender (18-6-0) promises "high-level striking, crazy speed and crazy creativity."
MacDonald and Millender are headlining PFL 2 at the Ocean Casino Resort in Atlantic City, N.J.
The Professional Fighters League, formerly known as the World Series of Fighting, works on a set schedule unlike other MMA promotions. The 2021 campaign, featuring six weight classes, started April 23 with each fighter having two bouts during the promotion's "regular season."