Janney was appearing on Tuesday's edition of The Ellen DeGeneres Show (February 16) after the sitcom was nominated for three Critics' Choice Awards.
"It was very odd, she is very missed," Janney said. "Anna is missed and her character on the show is missed.
"We are just happy she is doing what she wants to do now and we are happy that we are still able to tell these stories about these great women in recovery."
Faris announced her departure from the show last September, ahead of its eighth season.
In a statement, Faris thanked creator Chuck Lorre, the writers and her castmates for a "wonderful work experience" and allowing her to "pursue new opportunities".
Previously, Mom showrunner Gemma Baker spoke about why the show would never kill off Faris' character Christy.
"Although our show has killed characters off in the past, we never considered this as an option for Christy," she said.
"Our audience has been rooting for her for seven seasons and that loss felt like it would be too devastating for them and for our other characters. It was important to us that Christy moved on to bigger things.
"Quite often when people are in recovery their lives get bigger in ways they couldn't before have imagined. And so that will be the case for Christy. She has been working toward her dreams for a while now and we feel like the audience will be happy for her."
Mom airs on CBS in the US.
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