Compare the Celtics roster with the rosters of the league's top teams, Suns, Bucks, Heat, Bulls, Warriors, Jazz.
I left the Nets and Lakers out because they are essentially basketball pimps.
The Celtics are not facing a huge deficit in talent. They do appear to be trying to compete against superior coaching and more intense competitors who are mentally tougher and who play together. Celts may also be handicapped by weak ownership who love the spotlight, but not the work it takes to get there. If the primary reason, Wyc and Co. promoted Brad to the GM position was to not lose money on the expensive contract extension he had signed the previous summer, then we have real problems.
We don't need a huge upgrade in talent, though some would help. Mainly, we need players with different mentalities, a roster that compliments each other and a coach who can orchestrate those talents and effort into winning games.