"Well I'm Jen's -- one of Jen's -- best friends," he told DeGeneres. "I don't want to get competitive though. I get in trouble if I say that [Reese's] the best friend or you're the best friend."
DeGeneres responded to Theroux's hedging by saying, "But the honest truth is I'm the better friend."
"I don't know, she's pretty good friends with Reese as well. They're doing a show together," Theroux said before qualifying, "She loves you. She adores you."
"Let's make it equal friends," he ultimately decided with DeGeneres.
Variety reported in July that Aniston and Witherspoon will team on a new TV series focused on a New York morning show. The pair previously collaborated when Witherspoon guest starred on Aniston's last TV project, Friends.
"At this point in my career, I want to be part of wonderful stories, exciting characters, and also just having a good time," the actress said in January of her intent to return to television.
Theroux and Aniston married in August 2015 after meeting on the set of Wanderlust in 2011. The actress slammed pregnancy reports in an interview with Glamour in August, explaining, "It's just my body."