As one star returns to CSI, another is leaving the lab.

Producer Naren Shankar told TV Guide Magazine that Lauren Lee Smith won't return as the character of Riley Adams in the fall.

“She’s not going to be back for Season 10,” he said. “It was an issue of how we were feeling the ensemble was working. When we brought in the character of Riley, it got lost in the shuffle a little bit because we had lost Warrick, we had lost Sara, and then in the middle of last season Grissom finally left. I think, as a result, the character never quite found its footing in terms of the rest of the gang.”

As the result of Riley's exit, a slot will be open for Sara Sidle (Fox), who is now ready to return to work after her burnout in season eight.

“She shows up because the team is understaffed,” said Shankar. “The undersheriff reaches out for a recommendation and Sara says, ‘I think that person might be me.’”

Sidle will appear on at least five episodes, and Shankar assures fans that the character wll be featured prominently during this time.

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.