Danny DeVito and Rhea Perlman's split didn't just come as a surprise to their fans ... it also blindsided their close friends -- including "Cheers" actor John Ratzenberger, who claims he had NO IDEA the famed couple was calling it quits.Ratzenberger -- who played Cliff on the 80s sitcom alongside Rhea -- said he just saw his old co-star the other day and she didn't even suggest things were going south with her husband of 30 years.

Ratzenberger could hardly believe the news about the spit, saying simply, "That's sad."