"I've never interviewed anybody in my life, and I'm kinda tense about that and I don't want to screw it up, so I figured I'd have a little practice," the 40-year-old actor-comedian said on Tuesday night's show.
So he brought a good friend: his bulldog, Matzoball, who strutted confidently on stage and plopped down comfortably in the guest chair.
Matzoball's "kinda like the Ed McMahon of the show," Sandler said.
"So Matzo, how was your flight? Airlines (are) pretty nuts lately, don't you think?" he asked his pet -- getting no response. Matzoball stared at Sandler, then at the audience. For the most part, she seemed pretty bored.
Sandler had better luck with his other guests, who included Don Cheadle, his co-star in "Reign Over Me," which opens in theaters Friday.
"Unfortunately, Dave is not here tonight," Sandler said. "Just a little thing, we hope he'll get better real soon. He's suffering from the stomach flu."
He joked: "It's a common side effect from Botox injections."
Letterman, 59, has had extended absences following heart surgery and a case of the shingles, but this was believed to be the first time he had shown up for work and couldn't go on.
He will have time to recover. Letterman had already taped Wednesday's show, and he was being pre-empted for NCAA basketball Thursday and Friday.