Let me break it down for you.
1) Probably not worse than Ball or Mahaney. But all of them (+Stewart) rated horribly on D last year, so he's not unique in that.
2) His block and steal rates were good to fine.
3) The team's on/off defensive splits against decent or better teams were atrocious with him on the court (top 200 opponents).
Close to 14 points per 100 possessions worse with him on the court.
Why was that? Well opponents shot the lights out when he was on the court. From every area of the court. You adjust for 3pt shooting (un)luck and you still get 8 points worse per 100 with him on the court.
There is some further (un)luck with the mid-range shooting while he was on the court, but can't adjust that on this site.
So what was bad? The team fouled a ton with him and off, but it was 7% worse with him on the court, and his personal foul rate was high (close to 5 fouls per 40), which is atrocious for a wing. The team gave up shots at the rim 4.4% more often AND opponents shot 57% at the rim compared to 50.7% with him off. Those are all really bad things for a defense. Considering how often he and the team fouled, you'd think they would've forced some turnovers, but no, they forced less with him on the court.
These team-level stats do have confounding variables, obviously. The on and off don't have identical teammate constructions, for one thing. The stat RAPM does adjust for that and he graded as the worst defender on the team in D-RAPM. He played slightly more with Tarris than Samson, with a 57/40 split. On the season, Reed had the better on/off defensive numbers, so Ross played more with the better defensive center and still posted those horrible team splits. The lineup data shows that his minutes with Samson were particularly horrible defensively, especially the Ross/Stewart/Samson lineups.