Come on, man. He gets knocked off the ball like a rag doll, it's an issue of size and exertion. And you can't argue for a second that his stamina isn't lacking, he just needs to work on it.
I haven't singled Guthrie out, Colo is another one who needs to adapt his physical form, as is Nolan, clearly.
With Guthrie, it's a case of a very talented footballer being significantly restricted by something fairly simple to improve upon. Has there ever been a genuinely successful central midfielder in the history of the PL who couldn't impose himself physically? Doubtful.