I feel that a lot of criticism directed at Kemba is based on his play while he's still acclimatising to NBA basketball this season - he was a very good passer last year who blended quick decision making and off-ball movement with his on-ball prowess.
I’m not sure a player can be considered a very good passer when averaging only 4 assists per game, especially When that player is a point guard.
Two notable factors work to suppress Kemba's assist totals.
1) Touches. Kemba averaged just 70 touches per game last year. That is WAY below the number of touches that guys like Trae Young (87), Luka (96), Lebron (91) and other assist-per-game leaders average. Kemba shares a ton of touches with Smart, Tatum and others. Only 9 games in this year and he's been on minute-restrictions, but he's averaging only 62 touches per game so far this season. You are not going to generate high assist totals with that few of touches.
2) Offensive strategy. The Celtics last year and this year, given the particular types of talent they have, rely on isolation plays heavily. Their offense last year was designed around using 3-wing lineups to exploit match-ups and while they've lost Hayward, and have rarely had all their weapons available this year, they still are running the same strategy. Thus, the Celtics pass-to-create-the-matchup more than pass-to-finish. They do run offensive plays, but the result of the play is often just the match up, not the actual scoring attempt. This is why the Celtics as a _team_ have low assist numbers last year and this year.
But make no mistake, Kemba is an excellent passer. He led the Celtics in potential assists last year (8.2/gm) and maybe if some of his teammates were a little more efficient at scoring he would have had higher assist counts. He passed an average of 48 times out of those 70 touches per game (~69%) and as others have pointed out, his turnover rates were very, very low.
But with the above two factors in play, Kemba is never likely to post Rondo-like assist totals.
In this last game, the loss to the Suns, Kemba had 12 potential assists. His teammates, unfortunately, shot a lousy 2 of 12 on those plays. So he ended up with being credited with just 2 assists.