Earlier experiments on the effect of an external field on the nematic-isotropic transition temperature indicate its linear increase with field in the low field regime, in contrast to a quadratic dependence at higher fields. This was explained phenomenologically as due to the effective suppression of longer wavelength order director modes at low fields. We investigate this effect by carrying out Monte Carlo simulation on a lattice model based on Lebwohl-Lasher potential. We achieve the necessary high temperature resolution (one part in 10(4)) by performing an entropic sampling on the system, and our results support this phenomenological interpretation.