Grades for every NBA offseason signing: Why Boston's latest move is an easy A

The NBA offseason still features plenty of important moves across the league. I'm grading every free agent signing and the most impactful extensions, breaking down the ramifications for all teams and players involved. To determine each grade, I'm looking at multiple factors, including the player's on-court impact and age, the contract's financial implications and the context of the team's short- and long-term outlooks. How risky or certain is the move? And how much does it help or hurt the team's chance to win a championship, next season or beyond?