The match comes after Cena returned to Raw last week since moving to SmackDown Live as part of the Superstar Shake-up in April.
The matchup itself isn't surprising. WWE has been preparing for a Cena-Reigns clash for over a year after Reigns called out Cena on the Today show the day after WrestleMania 32:
The two stars also went back and forth on Twitter in July:
The timing of the match is somewhat surprising, though, since one would've expected the first marquee face-off between Cena and Reigns to take place at one of the traditionally "Big Four" pay-per-views (Royal Rumble, WrestleMania, SummerSlam or Survivor Series).
Putting Cena vs. Reigns on the No Mercy card, however, is a nice way to help build interest in the branded PPVs in a time when the importance of monthly PPVs has diminished.
WWE may also be unsure of how far ahead it can count on Cena being available to wrestle and wants to capitalize on his presence while it still can.
The 16-time world champion is taking on more acting commitments, which have forced him to take a break from the ring in the past. The Hollywood Reporter's Rebecca Ford reported in July that Cena received a role in Bumblebee, a spin-off in the Transformers film series.
Cena also told Complex's Khris Davenport he "[knows] that my days are numbered" in a WWE ring.
WWE already gave one dream match away on free television when Shinsuke Nakamura defeated Cena on the Aug. 1 edition of SmackDown Live. The fact Cena will meet Reigns at No Mercy could signal Cena's availability for WrestleMania next spring season could be in doubt.