Derrick White is a massively important player for the Boston Celtics. He steadies the offense, is the No. 2 scorer on the team behind Jaylen Brown, and remains one of the best defensive guards in basketball. At the same time, White's shooting efficiency has dropped pretty considerably as he's been given more offensive duties with Jayson Tatum sidelined.
This is pretty common, and White shouldn't be castigated for a drop in efficiency, especially with everything else he provides (and how good the team is anyway). In fact, White did the same thing in San Antonio.
Ironically, each year with the Spurs, when White's scoring went up, his shooting percentages went down.