The problem is defense. We are likely to face at least 2 of these team ; OKC, Portland, GSW & Houston. Green has shown be game be a great defender.
Westbrook has struggled against the Spurs in the last few meetings with Green seeing alot of time guarding him. He would also ,likely, guard Harden, Lillard, & Curry. Belinelli was a net negative player last season (His offensive rating subtracted from his defensive rating). Him starting would put way more pressure on Leonard, Splitter and Duncan to lock down their defender or lock down the paint.
Defenses also can't sag off of him. Even when he isn't scoring, he has a positive impact on offense. The best option is to keep Green and keep him starting. That or finding another 2 guard who can defend and shoot like him for $ 4 million and fits the franchises 2 year plan. That seems unlikely.