Tactical decision. He probably wanted CB on the bench and that's why we had Guehi in the squad. Can you imagine problems when everyone is fit? Let's say for example Kepa, Emerson, Rudiger, Tomori, Azpilicueta, Kante, Jorginho, Mount, Willian, CHO, Abraham starting 11. Willy, Christensen, James, Kovacic, RLC, Pulisic and Michy bench. That leaves Alonso, Zouma, Barkley, Pedro and Giroud out of the squad