Last season, Lexie (Chyler Leigh) and Jackson (Jesse Williams) began dating and Mark (Eric Dane) ended up giving their romance his blessing.
However, Rhimes explained that Mark will not have moved on from Lexie completely when the show returns.
"It's going to [remain] a viable triangle," she told TV Line. "Mark is trying to move on from Lexie, but there's inherent comedy there, especially as Jackson tries to pursue a career in plastics (Mark's speciality)."
Williams has previously revealed that he is happy about the romance between Jackson and Lexie as it means Jackson has a "confidante".
Grey's Anatomy will return with a new season on September 22 on ABC.
> 'Grey's Anatomy' to pick up days after season finale
Watch Jesse Williams's recent chat with Digital Spy below: