I think he was screwed when Boozer unexpectedly decide NOT to opt out of the last year of his contract. Millsap was expecting to become the starter after Boozer opted out, but when Boozer stayed, Sloan kept the status quo; started Boozer over Millsap. Kind of a bitter pill for Millsap to swallow... Anyway, Millsap needs to get over it, and I think he will, so long as his replacement actually EARNS the starting role.
I still think depending on matchups, which Corbin seems a LOT more aware of than Sloan was, Millsap can be effective logging some time at BOTH the SF and PF positions.