Looking at some on/off splits early season, there is bad news and there is good news for d00k lovers.
Bad news: Irving and Tatum haven't had a positive impact relative to their replacement minutes for the Celtics so far. Celtics are 0.5 points per 100 possessions better overall with Irving off the court. 0.1 better with Tatum off the court. The great Marcus Smart has been the best overall rotation player for the Celtics relative to his replacements, with strong offensive and defensive impacts. The Celtics are 9.6 better with Smart on the court. Similar story for Al Horford, as the Celtics are 9.5 better with Horford on the court, which is not too surprising, since their backup bigs (along with all backup bigs across the league) have severe flaws in their games. Aron Baynes has been the worst Celtics rotation player in this regard.
https://www.basketball-reference.com/teams/BOS/2018/on-off/
Good news: These stats are all messed up this early in the season, and shouldn't be taken too seriously. For instance, LeBron has been a negative player for the Cavs this season compared to when Iman Shumpert takes his minutes.
https://www.basketball-reference.com/teams/CLE/2018/on-off/