He's not terrible, when he gets into proper position and takes the right angles. At one point, his man caught the ball on the wing and Wagner ran at him, attacking him like he was gonna take away his left. Problem was, in that position, he should have taken away the baseline since he had help high but no help to low side. The player fakes left, dribbles right and has a wide-open layup. That's just dumb play. It's not inability to defend, it's just not smart defense.
Having said this, if you put Wagner at the point of your defense and let him get in position, he's pretty good. He has good quickness, good strength, and some tenacity. But he makes some dumb plays at times that are frustrating.