In such an unlikely scenario, I wouldn't necessarily trade him to the Knicks.
So the conversation would go something like this:
Kyrie walks into the office and say's "Danny I appreciate you pulling me out of that situation, and giving me the platform to show I could ball without LeBron but I'm signing with New York next year. I'm in the last year of my deal, you know my value isn't that high to 90% of teams in the league, trade me to the Knicks now and take back some assets as compensation for smoothing out the transition."
Danny: "I hate to hear that Kyrie as we are in the process of building something really special here in Boston, but if that's your basketball decision, and your mind is set, then I'll definitely look at trading you. I doubt the Knicks will offer anything near what your value is because they obviously know you want to play there. So, if they lowball us, then we will entertain other offers."
And at that point, best offer wins...whether it's Sacramento or Golden State (worst to first options
).
Bottom line: Again, in this unlikely scenario, Boston has no loyalty towards Kyrie and unless the Knicks are willing to part with adequate value for him in 2018/2019, then we trade him to whichever team can make the most of his talent as a rental and provide the best compensation.