Of Guz, Hill, Marshall and Ryu, Marshall seems to be throwing the best. It would only make sense that he is the one they would be thinking of as the 5th starter. Things may change between now and April 15th (when that 5th starter will first be needed) but as of now, no one is throwing better than Marshall. I'm not surprised. Guzman is rusty, Hill needs to work on his secondary pitches or become a LOOGY, and Ryu, while impressive in spring training, probably isn't as mature as Marshall and would be better suited for a half season in Iowa before coming to Wrigley. Also, given that Marshall didn't pitch that many innings last year, I think it would be better to use him as a fifth starter now and Ryu later in the season, if Ryu is needed.