Here's a positive take on this whole situation:
https://twitter.com/ByJayKing/status/721768000836210688
"When Avery Bradley didn't play against the Hawks on Nov. 13, Marcus Smart started and Celtics won, 106-93. Only W against ATL this season." 
i love smart but he's not better than AB.
IT is going to have to score 30 plus for us.
Well I really only posted this satirically, and no one said he was better than Bradley, though I think you can certainly make the argument that he's better than Bradley in every area other than shooting.
But I think people are reading too much into this injury. He happens to play a position that we're deep at and similarly skilled, so we'll be alright. We might lose some shooting in the offense with Smart, but you add size, better team defense, and better playmaking and passing in Smart.
Smart plays 30 minutes so maybe he gets another 6 minutes. So I think it's more about having to throw an inexperienced guy like Rozier out there or stretching out our starters and tiring them out. That's the part I'm most worried about.
The other part is, we have a few basic plays we run when things aren't going well. The Dribble handoff with AB shooting from just outside the free throw line area is one of our money plays. Not having that play will hurt the team.
This...
There is a very large potential right now for all the Avery Bradley haters to be silenced, because unless somebody else steps up in a HUGE way, we are about to learn first hand just how important he is to this team.
People tend to underestimate his impact, but Bradley is:
1) Our best on-ball perimeter defender
2) Our second or third best shot creator (after Thomas and maybe Turner)
3) Our second or third best outside shooter (after Olynyk and Thomas)
You take all of that away and it hurts..a lot.
But I'm hoping and praying that somebody can step up and help offset that loss. Smart was amazing in game 1, and we are going to need guys to step up like that throughout this series if we're going to have a chance. It doesn't necessarily have to be the same guy each game - it could be Smart one night, Crowder another night, Turner another night, Olynyk another night, Amir another night, etc. But every single night we're going to need somebody to step it up.
Bradley has been our warrior not only in the last game, but over the last 3 or 4 games while we've been struggling. Thomas hasn't been as good as usually, but he's still played hard and he's still put points on the board.
Somebody needs to help Thomas out, and the one man who is going to have a LOT more weight on his shoulders at this point is, to me, Crowder. Crowder has played poorly since his return from injury. The defence hasn't been up to his usual standard, nor has the hustle/effort. Hsi shot selection has also gone downhill. Crowder is such a handful when he is on his game, and we're going to need him at his best.
Smart is going to need to do his best to replace Bradley's defence and shooting too - I won't be angry at him if he struggles with his shot, because that's a natural weakness for him. But if he can find his shot and become an offensive threat in this series it'll be huge.
Turner has to be Turner. He has to be aggressive attacking the paint, and he has to make himself a factor in the series. He's got to take some pressure off IT as the ball handler and offensive creator, and I'm sure he'll do that.