Nobody would have blown up that young nucleus for a one year rental in Kawahi and PG, or a two year rental in JB.
Obviously in hindsight they should have moved Brown and Rozier for Kawahi, but KL was coming off a weird injury, had a meddling uncle, and was gone in a year anyway. Toronto was blowing it up anyway so to take a flyer on Kawahi was a no-brainer, but the Celtics had arguably the best team situation in the NBA 12 months ago. No GM in the league would have done anything to disturb that.