It sucks for Mahinmi.
First, while missing the first couple of weeks (mainly practices), wasn't a that big deal, missing all the preseason games sucks. these games were the best way for Mahinmi to show Pop that he was NBA ready. If Thomas, Oberto and Bonner are solid, it will be quite hard for Mahinmi to get a chance.
Second, Spurs also have to decide whether or not to pick the option on him for next year before the end of the training camp. I still expect Spurs to pick it because it's a cheap salary, Ian has shown good thins last year and they have nobody else in the pipeline, but this injury could give them some doubts about it.