The earliest evidence of the Healing Buddha is the Chinese translation from the Skt. of Bhaiṣajyagurusūtra (early 4th cent. CE), and as this has close similarities with the Lotus Sūtra (c.2nd cent. CE) in which the bodhisattva King of Healing (Bhaiṣajyarāja) is prominent, it is likely that Bhaiṣajyaguru was a development bestowing increased importance to Bhaiṣajyarāja. He presides over the Pure Land of the East (cf. Sukhāvatī, the Western Paradise).
"Bhaiṣajyaguru." The Concise Oxford Dictionary of World Religions. . Encyclopedia.com. (August 16, 2018). http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/bhaisajyaguru
"Bhaiṣajyaguru." The Concise Oxford Dictionary of World Religions. . Retrieved August 16, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/bhaisajyaguru