Yeah, but it was in a later season! Near the end of the series. There were rats, some even looking like Rizzo, but Rizzo was a full blown star after Steve Whitmire started giving him lines and clothing (Before then, he was...NUDE!!) I don't know exactly when, because I read he first met someone from Henson in 1978, and I think I read he tried out for Sesame Street. (In Of Muppets in Men, the book)
But in case you are wondering, Muppets Take Manhatten was not even his first film appearence. He was on Great Muppet Caper, as a bellhop! Infact, it was even a vocal role, because he sang a verse of Happiness Hotel ("So if you don't mind friendly animals, and you learn to stand the smell"). Hope this clears a little up.
If anyone knows more about this, please post.