Well I didn't see this coming and when I first heard the rumor I wasn't even sure it was permitted by the NBA. I guess the NBA's loosened roster rules due to the COVID-19 pandemic gives teams the flexibility to add players to the play-off roster all the way until their first postseason game. And since the Celtics win over the Wizards featuring Jayson Tatum's 50 point explosion technically doesn't count as the postseason, Danny Ainge used that leeway to add another big.
Hopefully the signing of Kwame Brown by the Boston Celtic isn't a sign that Robert Williams' Surf & Turf toe injury isn't more serious.