Who could have played at LW if Pep had started Kova instead of Alvarez? The logical answer is Foden. But that would mean to put Bernie at RW. Pep wanted Bernie in midfield as Kova is still new to the team. Kova takes more risks than Bernie as he loves to dribble through the middle.
I don't know whether Pep made a mistake, but Liverpool's weak link was the right side of their defence and Alvarez isn't a winger. Maybe Pep should have played Foden on the left and Alvarez or Nunes on the right. That was maybe a bigger mistake, if there was such at all, than going with Bernie in midfield rather than Kovacic.