I think Kanter's role is reliant on Theis and Grant and, for other reasons Robert.
Theis gets 22.2 mpg
Robert gets 14.5 mpg
Grant gets 15.3 mpg
Theis is a really good cog in the starter machine. He makes sure the three or four star players on the court with him most of the time can be stars. The Celtics don't need Theis to be anything more than what he is. He is 6'8" and 240 and can be pushed around by big centers, but, he is an experienced veteran who can survive. He keeps his minutes.
Robert is a first rounder and management has determined he needs minutes to further develop. Stevens is giving him 14.5 mpg vs last year at 8.5 mpg. Bob looks better and his fouling has gone down. He is not an offensive force, but, passes well, gets back on defense and is a bit better at his reactions and positioning. I don't see him losing many minutes to anyone. Be advised though? He isn't on the court at "important times"...ever.
Robert's little brother Grant also, needs minutes to become useful. On the Boston Celtics any player over 6'3" tall can be asked to play center at any time. It goes with wearing green, but Grant has been getting a minute or two at power forward or whatever you might call it. Unlike Robert, Grant has been on the court at crunch time and is a solid free throw shooter. He needs minutes and I don't see him losing any he's already got.
Kanter does have offensive chops, but, the Boston Celtic's starters have have some of the best offensive players in the NBA with the stats to prove it and if there are five guys who can play better D in close than Kanter?
Thus, your question about Kanter is answered by the three bigs in front of him, the four headed scoring machine that starts games and Steven's need for the young guys to play and improve....if for nothing else than to get traded by Mr. Danny.