I think his form since November has been over-praised, and his drop is visible statistically (assists is one measure, there's also total passes, long passes, key passes, dribbles, all down from previous seasons). But if used properly* he is a very scary and multi-dimensional midfielder. Short and long passing, dribbling, strength, flicks, crosses, tackles, defensive headers, good shot. A very rare package.
I think his first season was quite special (setting up many many chances with through balls for Ibra and others from a box-to-box position), his second was very good alongside Matic (same position). He has had exactly one spell as the attacking midfielder in a 4-3-3, in his third season with Ole as caretaker, and his numbers in that spell were stellar. This was Fred's first season, he was quite terrible, not sure what conclusions can be made about that. Since then he's been injured a lot, his output has been down.
Other than that one spell, he's never been in a 4-3-3, and he's always been behind a #10 (Mkhi, Mata, Lingard, Pereira) in a 4-2-3-1.
*4-3-3 with 1 DM, him and Bruno both attacking, rather than what he have where Bruno is level with the striker and Pogba hangs around with Maguire. I think it was obvious after Ole's initial half season that the way forward was to buy a Carrick-type player to anchor and dictate while he and Herrera/another go forward and create/press, but it was never tried, Herrera was sold, and we still don't have a defensive midfielder who can dictate the tempo of the game.