I don't see how there was any miscalculation here. They did not sign POB as a rotation player for early in the year. They signed him as a project, who could provide some length against certain teams, and with some coaching might eventually be able to earn more minutes (maybe not until next year). From the moment the signed him, they expected BBD to be the backup center, and then to likely bring someone in midseason like PJ.
I also think that the biggest reason he did not dress last night was simply because he was a horrible matchup against Houston. Their only guy over like 6'9" is Yao, and POB is way too weak to be able to defend him, while BBD is a great matchup, because he is able to keep him out of the post. All of their other big men, are undersized, but very strong. At this point in his career, POB is only really a good matchup against long, athletic teams. For example when we play Toronto, I fully expect him to dress, to give us another body at the end of the bench, who can match up with the length and quickness of Bosh, JO, and Bargnani.
The only thing that was miscalculated was some of the fans expectations, which were way too high.