Yes, and Faried played extremely well in limited minutes last year. To quote from SBNation: "Since joining the
Rockets, Faried is playing 93 percent of his minutes at center, after never playing more than 26 percent of his minutes there in a season before. That position change has coincided with a resurgence in his production, as his 14.3 points and 9.5 rebounds on 60.5 percent shooting in over 30 minutes a night thus far in Houston would all constitute career highs if they held long term. "
But Faried is just an example. All of the veterans who are still available have positives to contribute. They wouldn't be veteran NBA players if they did not.
If you don't think Grant Williams is a good as Kenneth Faried, why should the Celtics forcefeed him NBA minutes? Let him get minutes in Maine. And it may be that Williams becomes a better player by going against Faried or another veteran NBA big every day in practice.