Ray Allen, for the most part, has been MIA in the playoffs. Pierce has had his moments of brilliance, and Kevin played well, for the most part (except game 4). Part of the problem might be the offense that the team is running. But yeah, a "slump" this long is very serious. Something's going on here.
Unfortunately, we have to play who we have. Eddie House and Tony Allen are no better options than Ray Allen. House is not at all a decent ballhandler. Detroit will press him the moment he gets the ball, and Eddie will panic and turn the ball over. If Billups gets hot in game 5, then yeah, it may be time to get Tony Allen off the bench.
Some teams are explosive, they can run up the points very quickly. The Celtics, it seems, are not. Maybe they should have kept going to Kevin and Paul in game 4. The only way to get back into rhythm is to keep trying. Maybe they should have taken more chances and got Paul to continue shooting, hoping he'd get hot.