I would consider Brown, Tatum and Rozier to all be major X factors for us this season - it's hard to really put one above the others, since all three are going to have key roles now with so many of our other guys gone.
Terry Rozier
Rozier will be a key because in the past two seasons he's shown flashes, but has been extremely inconsistent - and I've given him a pass due to his lack of experience and lack of consistent role/opportunity.
However he's now going in to his 3rd season, and by default he is going to be our full time backup at one of the guard spots, so he's going to get a consistent role and consistent minutes. It's will be up to him to prove he can contribute on a consistent basis. If he can play the way we've seen him play at times, then he could be an absolute game changer for us as an Eric Bledsoe type guy - a small but hyper athletic guard who can create his own offense, hit big clutch shots, score in transition, and hound opposing guards on defence. If he continues to play with the same inconsistency then last year, then that backup guard spot could (on the contrary) prove a liability.
Danny seems to have extremely high hopes for Rozier, and so have I - but it's time for him to prove that faith is justified.
Jaylen Brown
Brown is also going to be a major X-Factor. We already know who 4 of the starters will probably be, which leaves one spot (either SG or SF) free in the starting lineup - and no proven starting calibre player to fill it. Most people seem to be predicting that Jaylen Brown will be that final starter. IF this is true, then this (holding down a starting spot on a team with ECF aspirations) is a LOT of pressure to put on the shoulders of a 20 year old, second year kid.
Will he rise to the occasion and excel? Will he try too hard to do to much and struggle? Will he still look green and make tons of rookie mistakes, and prove to be a liability? Honestly we have enough talent that we don't need to ask too much of him - with Kyrie, Hayward and Horford out there to carry the load and Morris for support, all he really needs to be able to do on a consistent bases is hold down his position defensively, and make open looks when he gets them. If he can do that as a bare minimum, that should be all we really need in order to round this out to be an excellent starting 5.
That said if he really breaks out and shows he can give us something in the range of 14 PPG / 5 RPG / 2 APG while shooting solid (44%/34%/70%) numbers and holding his own on defence (a feat that I do believe he's capable of)...then he could be the piece that changes this starting 5 for being excellent to elite.
Jason Tatum
A third guy who will be a genuine X-Factor for us is Jason Tatum. Normally, I would expect very little of a 19 year old rookie. I expected very little of Jaylen Brown last year, for example. But my expectations for Jason Tatum are a little different to normal for a 19 year old rookie for two reasons.
The first reason why I expect more then I normally would is that JT is quite possibly the most offensively polished rookie that I've seen come in to this league since Carmelo Anthony back in 2003. His footwork and vast array of moves is impressive beyond words for any rookie - it's mind boggling for a 19 year old. He already has better footwork and a wider array of offensive moves then most NBA starters, and that's no exaggeration - the kid can straight out score. He's going to need to bulk up before I can see him being overly effective as a small ball four, or even against some of the more powerful wings (Lebron, Butler, etc) but I see no reason why he shouldn't be able to score at a high level against 90% of the wings in this league from day one.
The Second reason why I expect more than I normally would is that in all of Danny's offseason moves, he traded away almost all our veteran players - and aside from three guys (Smart, Rozier and Baynes) our entire bench is made up of guys who have never played an NBA game. That means ONE of those 'rookies' is going to have to step up and make a consistent impact - and I don't see anybody on that list who is more capable of doing that then Tatum is. Smart and Baynes are both quite limited offensive players, and even if Rozier breaks out, we can't really expect him to carry the second unit's offense on his own - so Tatum's scoring ability is going to be critical to the success of that second unit. If he can come in as a consistent offensive spark plug off the bench (much like Evan Turner did in the past) then that could be the difference between our bench being a strong asset, versus out bench being a significant liability.