Fred does the CM role effectively, similar way to Tielemans let's say. He's not that good technically, but has better work ethic and energy levels, so it kind of evens out.
By "cover" I mean he can't play the deepest midfielder role, he has to have some support. It's quite easy actually. He does play that role for Brazil effectively so it's even easier to see what he brings there, hopefully we will see that soon with Casemiro joining. I also think he is useful in keeping the ball in opposition half, recycling possession and progressing the ball forward. I do think those are his strengths rather than making him play deep.
Fred is not even close to being great midfielder, but he's also a victim of being forced to do two man job more often than not. Fair enough if you think he's thrash, just keep that in mind. Many midfielders better than Fred would look out of depth in United team.
Pogba had no defensive awareness and no discipline to track back. Eriksen seems to have those, he's just not athletic enough (or maybe not fit yet?), so those are different issues to last season. I think Eriksen will be easier to accommodate in midfield than Pogba, I just don't see a point to it since that's not where he's most effective. Why can't we just play two defensive minded players in midfield and have the front 4 plus fullbacks do the damage? Everybody else does that, it's just we at United seem to have obsession playing AMs in midfield.