Boca's first-leg problem against Vasco isn't just that captain Leandro Paredes is out, it's that their recent games show two different Bocas: one that wins without the ball, and one that has 69% of it and still concedes as many shots as it takes. Against Instituto they had that possession and gave up 12 shots to 11 in a 2-2, which is the game Paredes was injured in. Vasco arrive after a 0-0 at Remo that their own coach partly put down to attention drifting toward this tie, so the question is who treats a semifinal first leg as a game to win and who treats it as the first half of a 180-minute job.
Boca first, from the match records in our DB. The 3-0 over Union de Santa Fe on Oct 3 was their cleanest recent game (25 shots to 4, xG 3.02 to 0.17). Seven days later Instituto got two goals from a game in which Boca had an xG edge of only 1.72 to 0.72. In the quarter-final against Sao Paulo the picture was split: 1-0 at home in an even game (xG 1.15 to 0.52), then 1-1 away with 37% of the ball, 7 shots to 16 and xG 0.71 to 0.88. So Boca have shown they can win without the ball and also that they can dominate it without much to show, and Paredes is the kind of player who usually links those two states.
Arruabarrena's own words point at the same worry. Ahead of the tie he said the team won't have Paredes' characteristics but has others, named Alarcon and Palacios as the ones who did well last time, and warned that Boca shouldn't overdo the intensity because the crowd pushes you into sendings-off (Diario Panorama, published Oct 12, which I opened). That is a coach planning for a first leg he expects to suffer in at times, not one expecting to run the game.
Vasco, from the same records: across their last six games with stats they average 1.8 xG for and 0.6 against, and the 0-0 with Remo was a 17-shot, 63% possession game with xG of only 0.84 to 0.25, so it was a blunt night rather than a bad one. Lance! reported that Brenner and Bruno Duarte travelled in the 25-man party, while Spinelli (foot knock in training) did not, so I'd treat their attack as a question mark until the team sheet is out. In the quarter-final they drew 0-0 away at Santa Fe and won 2-0 at home, which is the shape a team with the return leg at the Maracana would like to repeat.
What I'd watch is whether Vasco let Boca have the ball for the first hour and wait for Boca's pressure to run out of ideas, because that is the version of this game where the missing Paredes matters most. If Boca score first, the discipline Arruabarrena is asking for becomes the main story, since a tie plan built on finishing with eleven players is easy to say and hard to keep at a loud Bombonera.
