I disagree that it's a mild downgrade.
Khedira, Marchisio and Asamoah are all somewhat questionable in terms of performance, fitness and form. And no one should be surprised if all three are out with injuries simultaneously. Note: that I'm a big fan of all three as footballers, and I actually think that an in-form and injury-free Khedira is world class (highly underrated imo).
But this leaves us with Pjanic

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, Hernanes, Lemina, Sturaro, Marrone (?) and Pereyra (?). That's an appalling midfield.
Pogba will perform his best football then next five years (that's a huge loss transfer fee excluded), while besides Pjanic and Lemina our midfielders have all been at their best or currently at it.
We need at least one midfielder that matches the quality of Marchisio and Khedira. Whether it's a holding/defensive midfielder or a box-box, heck even an attacking midfielder, isn't all that important, since we can adapt with the players we got.
With the huge signing of Fatso, it seems rather foolish if we don't go for CL-glory this year and for that; we do need an european top class midfielder.