I feel like there is some difference in what is meant by a "role" player. To me, you are categorized as a role player if you are not good enough to be better than a role player. I know this probably is not all that clear but let me try to say that an "impact" player is a player that is not a role player. Our impact players are Walker, Tatum, etc (everyone's list will be a little different).
To me an impact player is a solid starter or a player off the bench that has the equivalent talent and production to a solid starter (Ginobili was an impact player off the bench for example). Not all starters are impact players. And not all impact players are created equal. Some impact players are stars, some not. Another way to think of this is that impact players probably play more than 25 min/game, rotation players more like 15-18 min or less.
To me, the impact players on the Celtics are Walker, Tatum, Brown, Smart, and Hayward assuming Gordon is back to full or near-full preinjury Hayward. I actually have Kanter as a impact player, admittedly fringe impact, but impact (I know many will disagree with that). All the rest are at best role players; Ojeleye, Theis, Poirier, RWilliams, Wannamaker are all role players, not good enough (based on to date performance) to be impact players. All the rookies (Langford, GWilliams, Edwards, Fall, ...) might maybe emerge as role players or even better but who knows. I know Poirier is technically a rookie but I put him with Theis, RWill, and that category based on his professional experience in Europe.
The issue for the Celtics is that all our impact players (with the possible exception of Kanter), essentially are "smalls". All our bigs (again, with the possible exception of Kanter) are at best role players. Not good roster balance. My hope is that one or more of the role player bigs elevates to an impact level but right now, none have demonstrated that level of talent or production.