Among those who have temporarily assumed the gig include Roy Wood Jr, Wanda Sykes and Brooke Shields.
Deadline understands that Clarksons absence was due to a private matter.
A spokesperson for NBCUniversal, which produces the Manhattan-based talk show, declined to comment.
Kelly ClarksonWeiss Eubanks/NBCUniversal via Getty Images
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Clarkson is preparing for a Las Vegas residency at The Colosseum at Caesars Place.
Her run is set to begin July 4 and will extend through Nov. 15.
The milestone episode will be taped March 18 at 30 Rock and air two days later.
