I suppose you can replace it with whatever will fit. If you don't have a derailleur hanger, you may have to get a hanger- or use the one on your existing derailleur.
What bike and derailleur are you speaking of, and what derailleur were you thinking of replacing it with?
__________________
*Recipient of the 2006 Time Magazine "Person Of The Year" Award*
Commence to jigglin’ huh?!?!
"But hey, always love to hear from opinionated amateurs." -says some guy to Mr. Marshall.