Anyway, about these Setien's ideas to play 3 at a back with 1 CB and 2 fullbacks, Van Gaal was doing the same often in his first spell from 1997-2000.
In 1997/98, we got Kod in a group stage after he lost 0:7 on aggreggate to Dinamo Kiev, and then in the 1998/99 season, he played 343 in our opening match at Old Trafford against Fregie's Man Utd: with CB Abelardo and fullbacks Sergi Barjuan and Reiziger as other 2 guys in a 3men defense.
A score was 3:3 and 3:3 vs Man Utd, but we got KOd from a group stage (Man Utd and Bayern went through), we lost 1:2 and 1:2 to Bayern in both matches.
We did play attacking and attractive with 343, but we were always on a verge of conceding 3, 4 or 7 against any semi decent European opponent.
Man Utd:Barca 3:3 goals from September 1998: