I consider Avery Bradley, Courtney Lee, Jordan Crawford, and most importantly Brandon Bass as role players who can make a difference.
What role do those guys play, though?
Bradley is a classic 3 and D guy. He brings the "D" every night, but his shooting is pretty streaky. He's not reliable as a floor spacer. Courtney Lee is pretty much the same in that regard -- only his defense is a little bit trick-or-treat, too.
Brandon Bass doesn't do anything particularly well other than shoot. Yes, he's become a pretty good defender in our system, but what he gives us in man defense he takes away in the inability to bother many shots around the rim (he's too short) or secure defensive rebounds. Bass has one defined role: shoot mid-range jumpers. That's fine, but he's been pretty streaky in that role this year and this team has a bigger need for other roles.
Crawford's role is supposed to be "microwave bench scorer." He's not bad at that role, but he's not very efficient as a scorer and there are a lot of games where he takes more away from the offense than he adds. Terry was supposed to do this, but quickly it was apparent we couldn't rely on him for that. Barbosa showed the ability to be this guy (as he had in the past), but then he got injured.
The Celtics really could use a "floor general" who calms down the offense and keeps things running smoothly. Andre Miller being the best example of that archetype. Terrence Williams in theory could be this, but he's just not experienced or very polished as a player. In years past this role has been less important in the playoffs because Rondo plays 40 minutes. Even so, Keyon Dooling was really helpful in this role last season.
The Celtics also desperately need a rebounding, do dirty work in the paint guy. Reggie Evans is the example of that archetype, along with somebody like Leon Powe or our old friend Perk. Jared Sullinger was supposed to be this for us, and was for most of the time that he played, but then he got injured. In theory we'd like Shav Randolph or Chris Wilcox to be this guy for us, but Shav, like T-Will, is very unpolished. Wilcox is just too soft outside of finishing lob dunks.
Finally, the Celtics really miss that "designated shooter." In years past it was Eddie House, and then Ray Allen became less of an all-purpose star and more of a super role player in this role. Jason Terry was supposed to be this guy, but he's been abysmal and anything but reliable. The Celtics don't have anybody on the roster other than Pierce who they can rely on for a difficult late three. Green has been pretty clutch shooting threes from the corners, but he's not hitting a last second three off a screen or a pick anytime soon.
You'll see the recurring trend here is that we lack these roles on our team mostly because of injuries, or because off-season acquisitions completely failed (see: Terry). This just goes back to the difficult truth that at the beginning of this season, we had the team we needed on paper.
That team didn't give us the results we expected for the first few months of the season, but maybe if they had stayed healthy they could have gotten it together for the post-season. But now, they're desperately undermanned and trying to make it work with spare parts.