We obtain a slow exponential growth estimate for the spherical principal series representation ρs of Lie group Sp(n, 1) at the edge (Re(s) = 1) of Cowling's strip (|Re(s)| < 1) on the Sobolev space H α (G/P) when α is the critical value Q/2 = 2n+1. As a corollary, we obtain a slow exponential growth estimate for the homotopy ρs (s ∈ [0, 1]) of the spherical principal series which is required for the first author's program for proving the Baum-Connes conjecture with coefficients for Sp(n, 1).