Celtics are up six games in the loss column over 9th place Toronto with 26 games to go. They'd have to fail spectacularly to miss the playoffs. Unless they fill the final two roster spots with Josh Beckett and John Lackey, they should be fine.
The Celtics are the definition of a middling NBA team - horrible on the road, vulnerable to late game losses, and decent at home. Get used to it, because that's how this regular season should play out.