The limit to high performances advanced scenario discharges with qmin above unity is generally set by the (2,1) MHD mode in JET. We investigate here the possibility that this mode is a (2,1) Neoclassical Tearing Mode (NTM) by computing the critical island width at which such mode would be unstable, using a non linear Magneto-Hydro-Dynamic MHD (MHD) code where the relevant bootstrap current physics is accounted for. We show that the triggering of a (2,1) NTM is consistent with a lowering of the critical island width as the plasma current diffuses towards the centre. This is explained partly by the increase of the magnetic shear at the resonant surface, which weakens the curvature stabilisation term, as found in the analytical framework of a Generalized Rutherford Equation. A comparison with experiment is made in the non linear regime, showing encouraging results on the dynamics of the confinement degradation and mode structure.