NBA free agency 2021: Why it made sense for the Boston Celtics to give Robert Williams III an extension

Trae Young, Shai Gilgeous-Alexander agree to max extensions (0:53)

Adrian Wojnarowski details the latest on Trae Young's and Shai Gilgeous-Alexander's massive contract extensions. (0:53)

The start of the 2021-22 NBA league year not only meant the beginning of free agency but also extensions for former first-round picks entering the fourth and final season of their rookie contracts.

The 2018 draft has provided a particularly plentiful class of rookie extensions. After Luka Doncic, Trae Young and Shai Gilgeous-Alexander all signed max extensions earlier this summer, the Boston Celtics became the first team to sign a 2018 pick to a non-max extension, locking up center Robert Williams III.

Which extensions look likely to age best and which might be mistakes? Let's break them all down here.

Boston Celtics

Agreed to a reported four-year, $54 million extension with center Robert Williams III