This may feel like an overpay, but Valanciunas is really really good. He has looked like the best player on the floor for the raps for stretches this season, and he advanced stats have been outstanding, his whole career and especially the last two years (career .166WS/40, season .211WS/40, playoffs .245WS/40). He's a traditional big man who isn't a prototypical pace-and-space guy, but I don't think that's an issue as long as we let Sully leave.
Valanciunas is a star, and he's locked up on a bargain 4yr/$64m contract. Ross also had a solid season in a limited role, and he provides positive value on his $10m/yr deal. I'd do this trade in a heartbeat, and frankly I think it's heavily biased in the Celtics' favor.