Some players can take on more games than others. I’m sure Unai and his staff know said players so has picked the team accordingly.
I don't think a small number of games in a few days is necessarily the problem, but more the continual build up over two or three months. We looked knackered after Saturday, but both games had been high-intensity for us.
Torres hasn't played many of the European matches, so has had week gaps between most games. Konsa came off after 60 minutes at the weekend. Could see Cash off after 60, Konsa to right back and Lenglet / Carlos on to keep things fresh.