Of course, the wrestling legend called out RR just last week ... saying he's coming for the Universal championship belt next month.
But, Roman doesn't seem too fazed -- ditching his shirt and soaking up the sun with his wife of 7 years, Galina Becker on Monday.
Just take a look at the pics -- the couple looks absolutely stunning ... with Roman showing off his tatted bod and Galina rocking a fun patterned one-piece.
SummerSlam is set to go down Saturday, August 21 at Allegiant Stadium ... and it's gonna be HUGE.
Cena -- a 16-time champ -- last competed at WrestleMania 36 in March 2020 ... where he lost to WWE Superstar Bray Wyatt.
Reigns ain't impressed by Cena's return, though ... saying, "He came out here, the same music, the same entrance, the same run to the ring, the same outfit, the same promo, the same insults."
"If I wanted that, all I had to do was search online for 2005 John Cena. It's the same thing, over and over and over. It's like missionary position every single night."
SummerSlam can't come soon enough!!