Waiters is a glorified sixth man.
Celtics don't need him right now.
He also would have been the best scorer on the Celtics last year. Something Boston desperately needs.
His scoring last year was roughly on the order of Bradley's.
per 36
Waiters 19.3 points, 36.8% from three, 45.5% from two, 68.5% from free, 1.12 points per shot
Bradley 17.3 points, 39.5% from three, 45.1% from two, 80.4% from free, 1.08 points per shot
Green 17.7 points, 34.1% from three, 44.8% from two, 79.5% from free, 1.18 points per shot
Thus, Waiters was 2 points per 36 better than Bradley and 1.6 per 36 better than Green. And Waiters did all this as his teams 2nd or 3rd option (once Deng was there Waiters was basically the 3rd option).
Waiters would have been Boston's leading scorer last year. He shot just as well as Bradley and Green did as well, so it isn't like you lose a bunch of inefficiency.