Let G be the group of rational points of a quasi-split p-adic special orthogonal, symplectic or unitary group for some odd prime number p. Following Arthur and Mok, there are a positive integer N, a p-adic field E and a local functorial transfer from isomorphism classes of irreducible smooth complex representations of G to those of GL(N,E). By fixing a prime number l different from p and an isomorphism between the field of complex numbers and an algebraic closure of the field of l-adic numbers, we obtain a transfer map between representations with l-adic coefficients. Now consider a cuspidal irreducible l-adic representation pi of G: we can define its reduction mod l, which is a semi-simple smooth representation of G of finite length, with coefficients in a field of characteristic l. Let pi' be a cuspidal irreducible l-adic representation of G whose reduction mod l is isomorphic to that of pi. We prove that the transfers of pi and pi' have reductions mod l which may not be isomorphic, but which have isomorphic supercuspidal supports. When G is not the split special orthogonal group SO(2), we further prove that the reductions mod l of the transfers of pi and pi' share a unique common generic component.