Sounds like it's painful for him to waste his prime years on the bench. they recognize his ability to change high level games so won't trade him. The B. Globe had an article too but I couldn't get past the paywall.
www.nbcsports.com

Brad Stevens addresses Payton Pritchard's desire to play more
Payton Pritchard remains with the Celtics despite expressing his desire for more playing time before the NBA trade deadline. C's president Brad Stevens spoke on Pritchard's comments and why he considers the young guard an important part of the team.