David Lee won't be "handed" a starting spot, but at the same time, he would not have been traded here if his agent hadn't been given some kind of assurance that the starting spot was more or less his to lose. If not the starting spot, then very significant minutes (28-30+).
Er, no. You're in Fantasyland.
He was getting traded regardless and there was no no-trade clause. There were only a handful - if that many - of teams who were willing and able to absorb him. Fewer still who actually could see him as a potential contributor. Lee and his agent had exceptionally limited leverage - like almost none - over the trade. He was getting moved, like it or lump it, because of the tax hit on Golden State.
As such, there is no chance whatsoever that Ainge and Stevens gave "assurances" to him that the "starting spot was more or less his to lose". There would be no reason for them to do so because Lee couldn't do a [dang] thing about it either way. If he pitched a fit, all he'd do is reduce his opportunity to play and hurt his chances of "rehabbing his value".
You act like he was a top free agent who can dictate such things. He's not. He's an aging, coming off an injury, overpaid 32 year old PF with limited appeal. He goes where someone is willing to pay his salary.