Because of the competition in the West, there is little chance Pops can risk resting Duncan down the stretch much, and for his plantar fasciitis to heal, he really needs to sit out a month at least. Even then, there is no guarantee it will heal. When Peja had it, it bothered him for months, then mysteriously went away.
Which, BTW, bodes ill for the Spurs come playoff time. This is clearly a weakness on the Spurs part that teams will try to exploit. It may work out early in the playoffs, but against Dallas or the Suns, I don't think so.