I've seen Warren play and have played him myself. He's a great player, but compare their rotation resumes on the big table and get back to me. There's no comparison.
No question, Mika has the best record and should be the favorite. But Warren is a 'A' level player, capable of beating anyone. NO ONE has to like drawing him. Warren has three top five finishes (3rd, 4th and 5th) in past U.S. Opens, with stronger fields. He has won major tournaments as well.