Along with the development of the theory of slice regular functions over the real algebra of quaternions H during the last decade, some natural questions arose about slice regular functions on the open unit ball B in H. This work establishes several new results in this context. Along with some useful estimates for slice regular self-maps of B fixing the origin, it establishes two variants of the quaternionic Schwarz-Pick lemma, specialized to maps B → B that are not injective. These results allow a full generalization to quaternions of two theorems proven by Landau for holomorphic self-maps f of the complex unit disk with f (0) = 0. Landau had computed, in terms of a := |f ′ (0)|, a radius ρ such that f is injective at least in the disk ∆(0, ρ) and such that the inclusion f (∆(0, ρ)) ⊇ ∆(0, ρ 2 ) holds. The analogous result proven here for slice regular functions B → B allows a new approach to the study of Bloch-Landau-type properties of slice regular functions B → H.