I think Fred falls within the group of players that fit the profile of signings that we needed, but are not quite working out (yet).
We needed a midfielder whose skill set compliments Pogba and Matic. That meant someone who is dynamic enough to cover for Pogba (but be less inclined to roam himself), technical enough to affect play from central midfield (without the space afforded to deep-lying playmakers or attacking midfielders), and ambitious enough to regularly attempt risky passes (even when they do not come off).
Fred did fit the criteria, despite not being the best in any of the categories.
From his limited appearances here so far, he does seem to be forward-thinking in his passing. He also tends to pick the right moment to play risky passes, but his execution has been average.
His close control has been a bit inconsistent, occasionally sublime but often heavy.
It's difficult to judge how well he's carried out his defensive duties, given how the team have defended as a whole this season, save to note that he is at least physically suited to play in this league, and that his combination play often lands him as the further midfielder forward. Whether you think the latter is a feather in his cap or a black eye probably depends on whether you are a Jose Mourinho.
At worst, his performances have been a mixed bag. Throw in the fact that he is a Brazilian player still adapting in a new league, it's surely too early to call his signing pointless. Also have to bear in mind that his performances are not the only reason for his lack of game time.
After the Wolves/Derby games, Mourinho chose to provide even more cover from midfield, by playing someone who can actually drop back and become an auxiliary centreback, and that Matic is not mobile and agile enough to perform this role. Since then, the third midfielder has been Fellaini (West Ham and Valencia) and McTominay (Newcastle), while Herrera had also been used in a similar role prior. You could argue that the team simply did not have a place for Fred.
Playing Mata was an obvious shift yesterday, with Fellaini and McTominay both unavailable. It was effective, against the least direct opposition we've faced so far this season.
With the benefit of hindsight, having an attacking midfielder was more effective in nullifying Chelsea's playmaking efforts than fielding either Fred or an extra defensive midfielder probably would have been. You can speculate whether that was by design, but Mata has generally done well in the limited minutes he's played this season.
But against more direct opponents, I would not be surprised if Mourinho returns to fielding a halfback, at least until he has more faith in his central defenders. That means Fred is going to find minutes hard to come by, but it doesn't mean he's not good enough to start for us.