Sandro, Toby, Savic, Bale still to be signed. Even if we do all that, are we a match for City?
Hell Yeah!
We would be a match for most teams in the Champions League if we signed the 4 you mentioned plus Fred and Dalot. The sheer competition for match day places would be similar to Sir Alex great sides of 1994 to 2001 and 2007-2011.
Imagine a Midfield that no longer relies on Pogba who on his day is a world beater but his days are far too few, Jose simply drops him and says for this game I'm going with Matic, Fred and Sergej Milinkovavik-Savic as I'm looking for more intensity. Against the top 6 away in the PL, Jose turns round to Sanchez and says your on the bench for this one because I need the extra pace of Bale, Martial and Lukaku to hit them on the counter.
Luke Shaw, knows Ashley Young is not better than him at left wing back but Jose simply prefers Young, if Sandro came in then Shaw has to really up his game because he sees a genuine class wing back that shares his physical strengths but makes better attacking and defensive decisions in game and will create more goal scoring opportunities. Finally Jose turns round to all the Centre backs and shows them Toby Auderweild positional play and long raking passes to open up other teams on the counter with pinpoint passing.
The simple test to the question is how many of this United's Hypertheortical Squad would then get into a combined City and United team.
Answer; 6 maybe 7 if Sanchez pulls his finger out!
D De Gea
(United)
Walker Kompany Auderweild Sandro
(City) (City) (United*) (United*)
Milinkovavik-Savic Pogba De Bruyne
(United*) (United) (City)
Bale Aguero D Silva
(United*) (City) (City)
*subject to signing for United as per your post you can see that window would give United the edge with Lukaku, Martial, Rashford, Bailey, Fred, Matic, Lingard, Mata all Elite Level back up players!!