I think this is very wishful thinking. With Onana, we will actually play higher in the pitch, which means that Maguire's weaknesses are going to be further exposed. In many ways, De Gea was Maguire's perfect keeper. Considering that De Gea was glued to the goal, it meant that we had to play deep in the pitch, which plays to Maguire's strength. Considering that De Gea also would not (usually) try risky and short passes but go long, it also meant that for most part Maguire's slow thinking in the ball wasn't getting exposed (we saw what happens when there are dangerous short passes to Maguire in the Sevilla match).
Onana is one of the worst possible keeper for Maguire and he is gonna look worse than he did with De Gea. I think his best bet is to go for some team that plays deep and is quite defensive. If Mourinho was interested, he would look good in Roma. In England, playing for Moyes' team would be good.
Expecting England's Magure is very wishful thinking. England plays very deep in defense, they usually had 2 defensive midfielders (Rice and Phillips or Rice and box-to-box Bellingham), and quite often 3 CBs (including a fast one like Walker). At United, he will just have one CB next to him and usually not that fast (Licha is slow, Varane and Lindelof probably a bit above average speed), and just one defensive midfielder in Casemiro, while probably being 10+ yards higher in the pitch. Also, Pickford is much more similar to De Gea than Onana (who is more similar to Ederson).