Roman Reigns was out of WWE due to pandemic since WrestleMania 36 and WWE had made lot of changes from then and replaced Roman Reigns with Braun Strowman and made him the Universal Champion. 

WWE thought that Roman Reigns will be back to WWE for Summer Slam and actually Planned him some big matches. But it seems he is not returning to WWE for Summer Slam too. 

WWE actually thought of involving Roman Reigns directly into the Title picture with Bray Wyatt and Braun Strowman for Summer Slam according to the Ringside News.

WWE had two options for him either to challenge the Champion for the title or to fight for the #1 Contender match for Universal Title. But Roman Reigns didn’t coming back to WWE soon.