An avatara is not a messiah. An avatara comes for a specific purpose. Rama came for killing Ravana and Krishna for Kamsa, Shishupala and Dantavakra. They would not have died in any other way. Ravana was more powerful than the lesser Gods (Indra, Sun and Moon, etc.) and ruled over them. Books say they were dangers to humanity. Of course, I am an atheist and I neither believe in avataras nor in Gods and Goddesses. It is not that all evil disappeared after Rama or Krishna and all humans went to heaven. The world went on in its way, but the gravest danger was removed.
@firedragon, in our view, Messiah, Avatar, or Manifestation of God are the same. So, for example, in our view, Jesus was the Messiah, or Avatar. In another words, in our view, Krishna and Jesus were the same Spirit, who appeared in different times and locations, with just a different body and name, but in reality Muhammad or Jesus or Buddha are the same Spirit, the Spirit of God, the Messiah, the Avatar, the Messenger of God, the Seal of Prophets!, the Buddha. All of these terms are related to the same Person. That Person is God, who manifested Himself, with different names, or terms, such as Messiah or Avatar!
That addresses the OP, in a more broad view. So, the answer to OP, from my view is, there is only and only One Messiah. Jesus, Moses, Krishna,... are that same Messiah or the same single Person, who had appeared, but just with an apparent different name and individuality according to the mission the Messiah had.
Ofcourse, most Muslims don't believe Hinduism, or Buddhism were given by Allah, neither the Hindus believe, the Quran was revealed by the Supreme Hindu God, but in Bahai view, all there religions, originated by the same God.