One thing that has been lost in the Boston Celtics' chaotic summer in the wake of Jayson Tatum's Achilles injury is that the team was likely on their way to losing their second round series vs the New York Knicks regardless. At the time Tatum went down, the Celtics were just a couple of minutes away from going down 3-1 to the Knicks, which is (in most cases) a death sentence for NBA teams.
Recently, Celtics legendary point guard Bob Cousy spoke on the frustration of losing to such a bitter rival, and a team that most experts thought Boston was many tiers better than, in the Knicks.
“…in the playoffs, in my experience, the best team wins,” said Cousy, per the Boston Globe, via Hoopshype. “There are upsets occasionally, but I have no understanding of how the Celtics could collapse, especially against the [expletive] Knicks. As the Knicks proved in the next round, they were not a legitimate contender.”
Indeed, the Knicks proved in the next round vs the Indiana Pacers that their formula of having their best two players–Jalen Brunson and Karl-Anthony Towns–both be clear negatives on the defensive end of the floor was not sustainable. It's still baffling that the Celtics were not able to take advantage of that fact.
A chaotic time for the Celtics

A couple of months after losing to the Knicks, the Celtics traded away two of their starters in Kristaps Porzingis and Jrue Holiday as part of an effort to get under the dreaded second apron.
While many pundits had forecast that major cost-cutting measures could be in store for Boston this offseason, the fact that Tatum will likely be out for the entirety of next season perhaps made that pill a bit easier to swallow.
Still, that doesn't address what exactly went wrong in the Celtics' series vs the Knicks, where they were fully healthy up until the Tatum injury.
Boston picked a heck of a time to have their worst shooting stretch of the season, and it didn't help that New York got some unexpected play from some pieces who wouldn't contribute much at all the next series.
All told, there have been better times in Boston.