ok, let's resume !
After finishing the grass upgrade at 7 October, I played 4 games, 7,8,10 & 12 and I had none injury.
Two power training and some small normal training and nothing.
So, grass upgrade didn't affect my injury rate.

Click image for larger version.  Name: 4 ames no injury.jpg  Views: 149  Size: 61.9 KB  ID: 55287