He'll get a chance next year, so all's good. 'Arry will just go out and buy another midfielder (either to replace the outgoing Modric and Bale or just 'cos he likes midfielders) instead of actually buying a decent striker.
Alternatively, Defoe's screwed because 'Arry is going to go out and buy Dave Nugent, Carlton Cole and Glen Johnson (to play up-front) and continue his tradition of buying players-that-really-aren't-that-good back from old clubs. Robbie Keane will also return, no doubt and he'll also make an audatious bid for Fernando Llorente, Diego Forlan and Sergio Aguero. Again.
Back on topic:
Defoe has himself to blame - if you can't score a goal unless it's from 30 yards as a striker, you're not especially likely to get a starting spot are you? Especially with the Abonimable Crouchie in the side.