Since Liverpool is not a weak team, I immediately think that Haaland should be fielded from the start of this match. There's no reason to weaken the squad temporarily because a tough opponent can always seize that opportunity as soon as the first half whistle blows. Doku might be a potential substitute to replace Haaland, perhaps around the 60th or 80th minute, to refresh the attacking line.
Actually I think Alvarez will replace Haaland because there are more players that can play in midfielder, Manchester City only have Rico Lewis and Oscar Bobb as substitute players lol. I have not yet see there's a club which lack of player because most of them are injured, so they only have 9-10 players which is not enough.
If I'm not mistaken, Haaland received an injury during this international break and even though the injury is not serious, there will definitely be doubts about not including him as a starter. Maybe Pep Guardiola will not force his flagship striker even if he has to play against Liverpool and indeed Alvarez will be a worthy replacement for Haaland.
Now the advantage is quite in favor of Liverpool because the attackers performed well in this international break and no one was injured, which makes them even more confident to steal points at the Etihad Stadium. I think with the number of squad that Manchester City has, it will be very difficult to stop every attack from Liverpool.