I don't think a dominant big man is as big of a need as a dominant wing player. There just aren't dominant big guys anymore and you can easily win with Al Horford types. But when the entire scoring load is put on one players shoulders and you have inconsistent, defensive minded players at both the 2 and 3, you likely won't go far. IT4 needs a player on the floor with him that can do more than just spot up shoot. Fultz or Jackson could be that player.
The conundrum that faces the Celtics is that there isn't a player in this draft who projects to have the impact of an Anthony Davis or Karl Anthony-Towns within the first year or two. I think you're likely to get a Jabari Parker or Andrew Wiggins level player who is kind of up and down to start and then starts to become established Year 2 going into Year 3, although still not quite as an elite player.
Basically I don't think Fultz, Ball, Jackson or who ever will be "starter on a Finals contender" good until about Year 3, if that, at which point Horford will be like 34, Thomas (a player who probably won't age gracefully) will be about 31 and alot of the depth that is currently on rookie deals (Smart, Olynyk, Rozier) will be gone or nearly gone, not to mention one or both of Avery Bradley/Jae Crowder as well.
So at that point you're left with:
1: Aging Thomas
2: Year 3 or 4 Fultz
3: Hayward if they sign him
4:??
5: Shell of his former self Horford, and not much depth because Thomas, Horford and Hayward are on gigantic contracts.
The main reason the Celtics are where they are (aside from how shit the East is) is their depth and the fact that Thomas, Bradley and Crowder are on extremely team-friendly deals. Thomas and Bradley will both be unrestricted FA's after next season and will likely both be looking for max deals.
Where they're at right now, next year, and maybe 2018-19 is probably their peak unless whoever they take at 1 surprisingly ends up being a transcendent player.
That's why I think trading it for an established star, if possible, is the best route to go.