I think a lot of the poor defender takes stem from the fact that he's not a shot blocker. He's not the guy who's going to sky in from the weak side swatting away shots. But his man defense has usually looked pretty solid to me.
And that was always the justification for pairing him with Turner. That Sabonis would benefit from playing alongside a shotblocker and Turner's outside shooting would help stretch the floor.
But the reality is that Turner is a good outside shooter but he also needs interior touches and plays better when there isn't another big clogging the paint and that Sabonis is a decent defender but shouldn't be chasing smallball PFs on the perimeter.
It's why Turner is having maybe the best season of his career statistically.
In terms of raw stats, Domas is actually a bit down from the last few years, but his per 36 and advanced stats look great. His TS% and eFG% are currently the best of his career and the numbers say he's been very effective on defense, especially the last few games.