Nathan Eovaldi virtually replaced Drew Pomeranz in the starting rotation last year with Eovaldi making his first start for the Red Sox on July 29 and Pomeranz his last start on August 7.
Outside of Pomeranz, Eovaldi and the Big Four, the Red Sox got only 25 starts from Brian Johnson, Hector Velazquez and Steven Wright for only 15.4 percent of the starts.
Steamer optimistically projects Chris Sale with 32 starts in 202 innings, David Price 32 in 194, Rick Porcello 32 in 191, Eovaldi 28 in 152 and Eduardo Rodriguez 24 in 136. Velazquez comes in at three and 67, Wright three and 69, and Johnson three and 18.
https://www.fangraphs.com/projections.aspx?pos=all&stats=pit&type=steamer&team=3&lg=all&players=0&sort=6,d
On the other hand, Marcel projects Sale with only 161 innings, Price 153, Porcello 176, Eovaldi 113 and Rodriguez 135. Johnson, Velazquez, Erasmo Ramirez and Wright are not far behind with 91, 77, 75 and 71 innings:
https://www.baseball-reference.com/teams/BOS/2018-projections.shtml
Many teams will get at least 25 percent of their starts from pitchers not among the preseason projected Starting Five. The Red Sox could be one of those clubs.