We can win the league with a Diogo Costa or Mike Maignan imo and dont need to sign Ramsdale from a domestic rival. Because to win the league with the current head coach and beat out a team like City to the title, who have elite level build up play, and a owner with very deep pockets. We need to feed the idea, so that the idea manifests itself on the pitch.
Without having strong first phase players on the ball, I can't see how we can beat City to the title. If our players in the first phase are better without the ball than they're on the ball, then that there is the issue. If your back 5, can't support the front 5 when it comes to playing the game in the opponent's half, then you've fallen victim to the opponent's press who have exposed your technical vulnerabilities on the ball hence a positional play coach like ten Hag has to make compromises in the build up phase.
If you want to see ten Hag's vision come to fruition, where we transition into a proactive attacking team, then we need players who are elite on the ball in every position, but particularly elite in the build up phase. And currently United have one too many players who aren't even close to being elite in the build up phase. So the key is to raise the technical level in the back 5 and deeper midfield positions.
De Gea isn't good enough for that type of football and has already been ousted by a champions league winning coach in Luis Enrique, who also wants his teams to exert zonal and positional control in possession.