According to Variety, John Cena is scheduled to begin filming Argylle in Europe in August. As a result, this could make it impossible for him to appear at Summerslam on August 21. However, there is still a chance that he could make the show, as there is no specific date for the beginning of the production, nor is it known if Cena will begin filming at the start. WWE has not officially announced or even mentioned Cena for Summerslam as of yet.
Cena has done some things at Summerslam before. Check out some of his efforts at the show: