Experimental observations show that the bN threshold for (2,1) NTM excitation is increased by flow shear, but the physical explanation for this trend is still unclear. In the present work, we investigate this issue by performing numerical experiments addressing the dependence of the critical island width on toroidal plasma rotation with the full MHD toroidal code XTOR [1], on the basis of a typical JET Advanced Tokamak case. We find that for situations where the Lundquist number is increased towards the experimental value, the (2,1) NTM is destabilized by flow shear at low magnetic Prandtl number Prm, while the threshold remains nearly insensitive to the flow at high Prm. The experimental trend is therefore not recovered, and possible explanations for this disagreement are discussed. A simple model of anisotropic viscous tensor shows that the high toroidal viscosity does not influence the value of the threshold, but comparison with experimental measurements suggests that the effective Prm seen by the mode is however larger than its small collisional value. Finally, the scaling of dimensionless parameters to ITER range is discussed.