The Red Sox heavily favor talent, and draft position. Guys like Xander and Jackie will get every opportunity, but others (mostly Dojji-type prospects) like Nava need to do their best at every single opportunity, or else they get stuck in the system.
Compare Britton and Workman:
Workman, 2nd round draft choice, 6.97 ERA in the regular season as a reliever. Sneaks into the postseason, 8.2 IP, 0 ER.
Britton, 18th round pick, 3.82 ERA in the regular season as a reliever. Franklin "Big Game" Morales got the nod ahead of him. Morales was the only member on the postseason roster who simply added no value to the team.