I don't think Havertz is being replaced. I actually think it's unlikely our striker signing plays more minutes than Havertz next season. At least in the short term, Havertz will be a better player overall than whoever we sign (barring an Isak longshot). And I also wonder whether we might benefit from more 4-4-2 if we had a proper box finisher in the squad.
We need a plan for parked buses. We don't need to play those games with a 6 and two 8's, which is what we've been doing this year with Partey-Rice-Ødegaard. Particularly with our defensive strength and use of inverted fullbacks, a two-man midfield should be enough to control the game. Havertz can keep the tactical/movement responsibilities of a #9, and then there's room for another forward with less responsibility, who hangs around the box looking to finish chances. Old school big man-little man. I'm not in love with Gyokeres but he could be the little guy goal poacher. Nwaneri might be able to play that role too. You could start certain games in this state (away at Everton, etc.), or substitute into it around 65'.
It's great to have the ControlBall capacity, it's made us really strong in big games, but sometimes you need to fit a poacher or a maverick into the team to make the difference. Fergie used to stack his teams with forwards who weren't the best all-around players but had a knack for scoring goals. United won a lot of leagues scoring off of scrubby rebounds.