Stevens is not one of the main problems with the team.
People are angry and looking for that one explanation for what's going on. Well, it's not that easy. The reality is that it's much more complicated. There are a bunch of things happening all at the same time:
1) The team dynamics have been affected by IT's personal tragedy. There's clearly an effect, and not just on IT.
2) The Celtics drew a team that's not a true 8 seed. The Bulls are better than their seeding or record indicates.
3) The Bulls are playing out of their minds. Doing great chasing the C's off the 3-point line, hitting shots they wouldn't normally hit, playing with incredible energy. The team is playing their best basketball of the year, while the Celtics are playing their worst.
4) The Celtics lack a second "alpha"-type and it's showing as things get tighter in the playoffs. Pack the paint and force IT to dish, but the problem is that the role players haven't been able to capitalize. That's one explanation for why they've been better in the regular season than the playoffs.
5) The Celtics lack size. Your starting guards are 5'9" and 6'2", and you have no one beefy at the 4 or 5 positions. You can get punished by a team with size.
It's not as satisfying as just throwing your hands up and saying "It's Stevens's fault!" or "Fire Ainge!" or "We shoulda got Cousins" or something. But this is the truth.