I don't understand what you mean by "replace Haaland". Do you mean get a backup for him, or should they replace him entirely? There is not a single striker in the world right now that can replace Haaland. As good as Alverez is, he couldn't do it. Only Mbappe would be able to do that, and if both are on the same team, Mbappe will likely play on the wing.
I don’t think anyone will even think about Haaland being replaced, if you check the post well, what the person is trying to say is that their should be a backup for Haaland Incase if anything happens to Haaland, then the back up player will take over, and it’s not really going have much impact on Manchester city performance whenever anything happens to Haaland. We all know that Haaland performance dropped last season, he performs better in previous seasons, but even with that there is no player that pep Guardiola can use to replace Haaland right now, but it’s really a good idea to have a back up player for Haaland, and Haaland shouldn’t be used in all Matches, he should be allowed to rest in matches that pep Guardiola knows that it’s not going to be difficult for him to win.