It's an awful heavy price for the 9th pick unless you think Winslow is really going to be a star, in which case 4 probable mid-late rounders is a fair offer. Some thought Winslow should have been taken earlier so taking that into consideration the Celts' aggressiveness is justified.
Still, there might be an opportunity later to package those picks for something else valuable. Understandably, the Celtics felt a lot of pressure to do something big because they can't keep drafting so many young players each year, the roster size won't allow it. They may also know they won't be picking in the early lottery anytime soon and this was a shot at getting a top talent at a relatively late slot.
At some point they have to trade some of these middling picks and OK-but-not-great guys on their roster and acquire 2-3 actual star players. So far, nobody is falling for their quantity-not-quality pitch though.