I think we all should get used to this type of result. This is a decent team, but nothing special. These guys will get the occasional big win against a top team, like the recent win over Milwaukee, but that sort of thing happens for most teams over the course of a full season—good teams have occasional bad days, and bad teams have occasional good days, and sometimes the two things align. Most Celtics games, however, against the upper-tier teams will be like this one—close, but no cigar (think the Utah game, then Golden State, and now Philly). They'll have some valiant play and some inspired comebacks, but usually they'll fall short.
Why? Bad health and lack of top-end talent.
This team cannot get healthy and stay healthy; they've proven that year after year, even before covid. And even if it somehow pulled off great health, it doesn't have enough firepower—just two pretty good players in Tatum and Brown, followed by a bunch of inconsistent role players, most of whom can't consistently shoot their way out of a paper bag. So unless Tatum and/or Brown becomes a top-5-ish player, or Boston acquires a high-impact player to go along with those two—and Boston finally gets a run of good luck on the health front—games like tonight's are going to be the rule rather than the exception.