Certainly the Phillies and Harper are free to change the plan, even if it disappoints thousands of Iron Pigs fans. I just don't think they will. There's no reason to rush him to face the Pirates (especially as he likely would still have Sunday off) and they were also very conservative with Segura. His first couple of games will be about how the injury feels (pain, range of motion), the next three will be about finding a groove.
If he goes 10 for 10 and leaves on Friday the Pigs fans would just have to live with it; would actually be more frustrating to have tickets for, say, Thursday, and he needs a day off (or only PHs) due to soreness.