If Rodri and Cancelo were in top form, they'd be starting in that game. Both were great during the run of 21 wins but have lost a bit of form recently.
As things stand, Zinch will probably start, Ferna may start as well. IIRC, Ferna struggled a bit vs Chelsea at Wembley, wasn't his best game, hopefully his problems were related to our weaknesses there (many squad players, etc.), and not to Chelsea's strengths. Rodri had problems vs Chelsea, especially at the Etihad, but he was the only proper midfielder then...Trust Pep to take the right decision.