The effect of a small Gaussian shaped deformation on the development and growth of Tollmien–Schlichting disturbances on an unswept airfoil is investigated. A broad range of gap depths and widths is modeled that can be sufficient to generate localized pockets of reverse flow. Boundary-layer profiles are computed using a Navier–Stokes solver, permitting a thorough investigation of all configurations considered. The linear stability of Tollmien–Schlichting waves is then examined using parabolized stability equations and linearized Navier–Stokes formulations, with the former method giving excellent agreement with the latter for all disturbances studied, including within separated boundary layers. Tollmien–Schlichting disturbances are amplified by deeper and wider gaps, with a correlation derived by relating the increase in the N factor with the gap dimensions and freestream Reynolds number.