We have studied the reaction K+n-->K+N*0-->K+π-p at 12 GeV/c using data obtained in an exposure of the deuterium-filled SLAC 82-in. bubble chamber to an rf-separated K+ beam. The M(pπ-) spectrum exhibits a very large enhancement below 1.8 GeV which we identify as primarily a diffractive effect. A partial-wave analysis of this enhancement shows that the data can be interpreted in terms of P12, D32, and F52 contributions. The P12 contribution which dominates at very low momentum transfers appears to peak around 1.25 GeV. If this state is associated with the Roper resonance, a mechanism which produces a downward mass shift of about 200 MeV for the diffractively produced pπ- decay mode is implied.