But he's not - IMO - a "squad/backup/reserve/2nd string player."
B. Silva is currently played mostly on the wing where he doesn't fit at all.
I think Pep bought him to play centrally - but at the moment we have so much talent there that he doesn't make the squad in spite of his talent.
I'm not saying that B. Silva will succeed for us - I think he will but it's not certain - I am saying that I don't see him as a winger in our side.
I agree with all you just said but I think of Mahrez in the same way. Not a guaranteed starter by any means but not "2nd string".
1st choice: Ederson, Walker, Otamendi, Kompany, Mendy, Fernandinho, Silva, KDB, Sterling, Sané, Agüero
Rotational options: Gündogan, Bernardo, Danilo, Delph, Jesus, Laporte, Stones
Back-up players: Bravo, Zinchenko, Foden, Díaz
For me, Mahrez fits in to that second list.