Its my understanding from reading an interview with the actor who played Gus, that the fifth season starts in 'real time', meaning right after the explosion.
Ok so here's my guess:
Anyone remember Mike? Gus' tough guy private eye? Last we saw he was recovering from the poisoning/ shoot out in Mexico inside that tent hospital.
Makes total sense for him to re enter the story now, either to evenge Gus, or play some role in running the distribution with or against Brian Cranston's character.