ERA isn't the only measure of effectiveness. Sheets has a better WHIP (1.04) than Volquez (1.17). In fact, Hamels, Sheets, Wainwright, Dempster, Webb, and Peavy, all have better WHIPS than young Mr. Volquez. Volquez does lead in K's but is K/BB ratio of 2.44 is well below Sheets' 4. You might start a rookie if no veteran is deserving, but there are several veterans having outstanding years, including Sheets. Another factor is availability, and Sheets figures to make starts on 6/29, 7/4, and 7/9, meaning he'll have 5 days of rest for the July 15th game. Volquez figures to start 6/26, 7/1, 7/6, and 7/12, so he'll have 3 days less rest.