I watched the recap of the MIL / BOS game on Hoops Tonight with Jason Timpf on YT. He's really knowledgeable and calls it like he sees it. He looked at the numbers in the series and the game film, and the numbers show that Boston is way more efficient in a half-court offense than Milwaukee. The only way the Bucks have been competitive is from BOS turnovers and breakaways. MIL is averaging 2.0 - 2.3 points per turnover / transition scoring on fast breaks (Boston scores at about half that in similar situations). The rest of the time though, the Celtics are significantly more efficient than the Bucks. In his opinion, if Boston holds onto the ball and just slows the game down into a half-court game, Boston has been significantly better so far (without Middleton playing) and has a huge advantage.
One thing I found interesting is he reviewed that the Bucks always have Lopez patrolling the paint and Horford goes and camps out on the perimeter . . . meaning that Al is frequently open for clean looks from deep. Unless and until the Bucks come out of the paint, those shots will continue to be there for Horford and Williams. But that's not how MIL plays defense, and their defensive philosophy is to protect the rim. Up until now, Boston has been making those shots (and the Bucks players haven't), and the size of Horford and Williams has allowed them to get their deep shots even if contested by smaller players.
Timpf went on to say that the hope for MIL is that Giannis plays even better (probably unlikely since it seems like he has run out of gas by the end of games as the series has gone on) and that the supporting cast can start hitting their threes. He doesn't expect those things to happen, as Giannis has taken so many shots that the other guys can't get into a rhythm and Holiday isn't accustomed to having to be relied on to take 25+ shots each game. Timpf predicts BOS to win by 10-15 in Game 5 and to win a close game slog in Game 6. Bottom line for him, if Boston doesn't get reckless with the basketball, they will win the series.