Unless Keyon Dooling's deal was non-guaranteed, the Celtics are brushing very close to the hard cap line. White makes $400K more than Melo, and they simply might not be able to afford it and maintain any flexibility to add a vet for the minimum in February or March. It may sound silly, but I'm not certain that Royce White is worth that extra $400K at this point.
I also think that there is value in trying to develop an athletic 7-footer, even if he doesn't offer much for the next two years.