"[Petersen] told me he's going to come back this season to help us with Marg's exit," series star George Eads said.
Helgenberger is set to exit the series in January.
The last time Petersen made an appearance on 'CSI' was in surprise cameo in the Feb. 2011 episode 'The Two Mrs. Grissoms.'
In other casting news ...
Dan Lauria has booked an unsavory 'Law & Order: SVU' role. Lauria, known to a viewers as Jack Arnold from 'The Wonder Years,' will play a youth basketball coach suspected of being a sexual predator. 'Necessary Roughness' star Mechad Brooks, rapper Heavy D and basketball players Carmelo Anthony and Chris Bosh will also appear. [TV Guide]
Dylan Baker is returning to 'The Good Wife.' Baker will reprise the role of Colin Sweeney when Alicia needs his help on a case. Look for the episode in October. [TVLine]
Nina Kronjager will guest-star on 'Covert Affairs.'The German actress will play an ex-girlfriend of Arthur. [TV Guide]
Lily Rabe is joining the cast of 'American Horror Story.' Rabe will play the original owner of the house the main characters buy. Her story will be told via flashbacks to the 1920s. [E! Online]
Cobra Starship plans to rock '90210.' The band will appear in a Rock the Vote episode of The CW show in October. [EW]
Harry Groener, aka the Mayor on 'Buffy the Vampire Slayer,' will guest star on 'Once Upon a Time.' He'll appear in the first episode Jane Espenson is writing. [Jane Espenson via Twitter]
'Dawson's Creek' actress Meredith Monroe is moving to 'Hawaii.'Monroe will guest star on 'Hawaii Five-0' as an unhappy wife who steals from her husband, only to find out the money wasn't his. Kidnapping ensues. [EW]
It's a family reunion on 'NCIS'!Robert Wagner will reprise his role as Tony's dad, Ralph Waite will be back as Gibbs' father. Plus, Abby's getting a brother. [TVLine]