Centre is a way easier position to defend than Wing. The only thing wingers had going for them defensively in the past was that they had other wingers running at them. But we all know what the modern day winger can do in attack.
It's the hardest position to cover defensively on the feild. Hence the percentage of tries scored in those left & right channels.
You not only have to be up to it one on one technically, but you also have to cover kicks & come up with decisions other positions don't have to worry about. They don't see as much defensively as the rest of the team but what they do have to deal with is full on make or break stuff.
Saying Grothe isn't up to it defensively is crap. He used to misread the plays & come up with wrong choices.... He hasn't for a long time, & his one on one is great. Wingers often get made to look like mugs because they miss they last tackle... A tackle that most often should have been made infield had the line pushed up, or had the markers not been bludging. Or when a two/three on one occurs because a centre, or back rower rushes out of the line & creates an overlap.
He'd cope defensively. He'd probably have a feild day actually.
That said I'm not suggesting we play him there. We have better options in the team that allow him to be better used in his best position. If we didn't however, He'd handle it & then some!