If you're a proponent of bringing back Mahinmi, I think there are 2 things you genuinely need to be concerned about.
First of all, the Spurs have made it clear that they intend to go hard to try and acquire Tiago Splitter. The way I look at it, having a frontcourt rotation that consists of Duncan, McDyess, Splitter, and potentially Bonner essentially relegates Ian to the end of the bench, leaving him with little to no playing time. This is especially troublesome since Ian, though he has great potential, is still somewhat raw and his deprivation of minutes, which is only compounded by his lack of D-League eligibility, will only derail his development.
Second, and I think most people fail to consider this, is that other NBA teams actually seem genuinely interested in him. As stated above, Ian, with his astounding physical tools, has, at least in my opninion, quite a high ceiling, and an NBA team may, as is usually the case with free agency, find a way to make him an offer that may put him outside of our price range.
Looking at things from this perspective, it's hard to see Mahinmi being back in the Spurs next season.