If it doesn't happen, then you...
(1) Re-sign Kyrie at 5/188M (max)
(2) Let Rozier and Morris walk
(3) Use the MLE on a big man like Robin Lopez (or someone similar that's available in FA)
(4) Sign another guard for the bench using an exception (like say, Isaiah Thomas lol, or someone else)
(5) I think we'll also have to decide on Theis since he's a RFA, but I'd be fine bringing him back if it's for relatively cheap. Do we have his Bird Rights?
(6) Hope Memphis Pick DOESN'T convey. Let it roll over for 2020 or 2021. Use 2 of the 3 picks (Kings, Clips, Celtics) on best player available at the range it's in, and trade the other for maybe another future (likely protected 1st round) pick since I don't think we can just fit 3 more rookies into our already crowded roster (probably just 2).
I think AD ends up with the Knicks or some dark-horse team like Portland. Ainge isn't dealing squat for a rental who doesn't want to be here. But we're fine as is, especially if our young guys keep growing and we can maybe land 1-2 contributors in this year's Draft.
I think it's ridiculous to think that Kyrie's decision will simply be based on if the C's land Davis. In a way, that makes it sound like he holds all the cards and not Ainge (which is an ugh feeling).
If we unfortunately lose Kyrie, then we take a small step back but we still keep growing with Tatum, Brown, Timelord and others we draft in the future.