I'd say he's a far superior athlete to LJC (especially post injury) and would be the best on the team by a decent margin:
I think Grant would slide right into that spot Simmons currently occupies. In the very least, he seems like a decent (and very willing) help-defender/rim-protector, and seems adept at finishing, which is something that will be needed if Dedmon is indeed the starter. His perimeter skills seemed to have decline a bit since his rookie season. He was pretty much able to dribble-penetrate and spot up during his first year, and both of those things seem to have tapered off somewhat during his sop re campaign. Depth behind Manu would be a little sticky, but Murray and whichever guard the Spurs keep (if not both) would be there to provide emergency filler. It's not ideal, but neither is relying on Simmons to be a rotation player.