Ritter has returned to the show to reprise his role as Mark, a love interest for Sarah (Lauren Graham).
However, he told TV Line that the return of Sarah's ex Seth, played by John Corbett, may cause problems.
Ritter said: "She's trying to balance Mark and her new life with this man that she has very complicated feelings for. This is the father of her children and she loves him in certain ways.
"But she also wants to start anew, and she's not sure that Mark can take on everything that's in her life."
Parenthood continues on Tuesdays at 10/9c on NBC.