According to The Dirty Sheets podcast, via Sportskeeda, John Cena will indeed be facing Baron Corbin at WWE SummerSlam, following the events which transpired off the air on Smackdown Live this week.
As noted, after WWE Smackdown went off the air, Baron Corbin hit the ring and attacked Shinsuke Nakamura, prompting John Cena to get involved. Cena and Corbin then brawled to end the show for the live crowd.
The report below adds, as we noted before, that the plan for John Cena after SummerSlam is for him to move to the Raw brand full-time: