Walker has been largely brilliant, but he fell asleep for Lyon's 1st goal and played their forward on. Also, he wasn't exactly faultless in the 3:0 loss in Liverpool.
The more games you play, the likelier it is to make mistakes. Dias wasn't faultless in Madrid last year, he made a mistake in Munich 3 weeks ago, which Sane failed to punish. Both Dias and Stones were average in the CL final, but people tend to blame Pep for it (no DM!). Manu had a terrible game in the Brentford loss..etc. No one is perfect.
In fairness, Laporte has made more mistakes in big CL games than the other defenders. He is likely to be on the bench anyway. Akanji was excellent in Munich and deserves to start. Walker will have the most difficult task tomorrow. Guess Stones will have to help him. Dias will take care of Benzema.