Drafting a guard shouldn't be considered an issue for the Celts.
In two years they'll have a decision to make on Isaiah and Bradley. My guess is they keep one and let the other go.
As for Smart and Rozier, both are expendable and likely to be included as sweeteners in any offer Ainge makes in trade discussions for star caliber players.
Even if the Celts re-sign Isaiah, as he ages he'll have to play fewer minutes and carry a lighter load. Having a top young guard prospect backing him up or playing alongside him wouldn't be a bad thing, and point guards typically take 3-4 years to develop to the point where they're prepared to lead a good team, anyway.