Both feet on the ground and it was his shins, not studs, that caught the player. He had gone off the ground though.
The way he dived in was so wild and out of control - both feet left the ground - means a red card was correct.
Did you see the replay? It wasn't wild and it wasn't completely out of control. It's a definite yellow card, but not a red, for me. You see worse tackles than that all the time.
But he did not go into him with studs. His feet had gone back to ground and he had pulled his feet back to avoid going in studs first. It was a stupid tackle but a yellow would have been sufficient.
Ref has to deal with these elbows. If Xhakas challenge was bad enough for a red card so where the 2 elbows into Mustafi's face.