I was happier thinking it really was Michael Jackson playing a mental patient playing Michael Jackson, thank you very much:

Michael Jackson hand-picked an MJ voice impersonator to sing the famous "Happy Birthday Lisa" song featured on "The Simpsons" back in 1991 ... this according to Lisa herself, Yeardley Smith.

Smith was outside the Red O restaurant in L.A. this weekend, when she explained, "[Michael] was not allowed to sing on the show, so he literally hand-picked a guy to sing like him."

During the famous episode, "Stark Raving Dad," Michael provided the voice to a mental patient who thought he was MJ ... but according to reports, Jackson's record label blocked MJ from singing the Lisa birthday song.

MJ ultimately selected a man named Kipp Lennon — and Yeardley explains, "How unnerving would it be to sing like Michael Jackson in front of Michael Jackson?"

[TMZ]