I have only watched Silivar a couple of times but he is so bad at so many things I'm sceptical he can become as good as they hope. Some people were saying he was unlucky to have that ball deflect off him in mid week but he dived so late he was kind of crawling along his goal line. He just seems to have really poor technique in so many areas.
He is 18, never had played professional football before he arrived this Summer, they signed him from Anderlecht because he didn't wanted to renew his contract with them, he still has a lot of matches to develop.
Main problem I can see is their former GK coach went to work with Marco Silva and their current one from the stuff I read in Benfica forums isn't much rated, only time will tell if he is going to improve.
Oblak with the same age was loaned to Leiria, the year after to Rio Ave, and only established in the first team with 20 years.
Ederson has the same age as Oblak, when he was 18 Benfica loaned with him to a club in the third tier in Portugal, below the level of the B team, spent the other 3 seasons at Rio Ave until we was 22, when he returned to Benfica he wasn't the number 1, it was Julio César the former keeper from Brasil NT, only played as Benfica number 1 for a year and a half.
This all to say, if you ask me as a rival do I want the kid to keep doing the same mistakes? Absolutely. Is he better than Patricio? Of course not. Will he ever be? Don't know, the Benfica scouts were keeping an eye on him since he was on the Anderlecht youth teams, and also on a under 17 European Championship where he played for Belgium.
Of course this only was possible because Julio Cesar doesn't have the capacity atm to replace Ederson, Bruno Varela former Portugal under 21 keeper simply doesn't have the potential, so the kid was launched as number 1 keeper in a team who has the biggest suport in Portugal, in a awfull context where they lost 5:0 at Basel and with CSKA at home, and the first game he played was a Cup match where they played against a club in the third tier and were totally dominated, and 4 days later he played his first competitive match at Estadio da Luz against United.
How many kids with his age do you see playing as keepers at this level? In the Champions League? Maybe he is not yet ready, but certainly will improve, different is trying to compare him with Oblak or Ederson, different context and situation, only in 2 to 3 years we can see if he is going on the same path or no.