The 5 first team players that have walked out the door in the past 12 months scored nearly 400 and assisted 180 of the 864 goals we've scored under Pep.
Haaland is a genuine superstar, but on his own he can't make up for losing all 3 of the biggest goalscorers we've had under Pep. Alvarez has to step up from pretty much day 1. He doesn't have to be world class, but he needs to contribute.
You are right to point out that we've lost a lot of goals with the departures of Aguero, Torres, Jesus and Sterling but it's worth bearing in mind that Aguero wasn't here last season and only contributed 4 league goals and 2 Champions League goals the season before. Despite that, we still managed to win back to back titles and were seconds away from reaching consecutive CL finals.
A more helpful way of looking at it IMO is to concentrate on the goals we've lost from last season.
Sterling - 13 PL, 3 CL
Jesus - 8 PL, 4 CL
Torres - 2 PL, 0 CL
So we've lost 23 goals in the league and 7 in the Champions League from the season just gone.
Haaland may not cover all of those himself but I suspect he won't be far off.
Then look at what our other forwards got and there's plenty of room for improvement.
Foden - 9 PL, 3 CL
Grealish - 3 PL, 1 CL
Mahrez - 11 PL, 7 CL
Palmer - 0 PL, 1 CL
I think we can expect a higher output from all four not least because they'll have more game time due to Jesus and Sterling's exits but they should also all benefit from the attention Haaland will be getting from defenders.
Foden will score more now he doesn't have to keep dropping deep all the time, Grealish will surely do better, Mahrez is capable of at least replicating last season and anything Palmer gets will be an improvement.
Let's say Haaland gets around 20 goals and the four mentioned above (less so Palmer) can improve on last season then I don't think we need to put too much pressure on Alvarez. We have to allow him time to adjust to a new country, language, league, club, system, teammates etc. Personally I'm just hoping he can chip in with the odd goal and anything else is a bonus.