Wow. I just checked his on/off splits this year and they are AWFUL on the defensive end.
Opponents average 101 points per 100 with him off the court...and 112 with him on.
EFG% goes from 48% (off) to 54% (on).
And get this, opponents Reb% is 47% with him off the court, and 49% with him on the court.
Overall his net on/off is a -12. The Pistons outscore their opponents by 6 pts with him of the court, and get outscored by 6 with him on the court.
Some of his other defensive metrics look good, and I'm not an expert on the Pistons, so maybe there's some explanation for the above. But those numbers do not say "blow your salary cap room on this guy."