I have no problem with us selling Fred at some point in the future; perhaps in a year or two. To date, he has offered almost nothing in the defensive third of the pitch and likewise in the attacking third. Virtually all his contribution comes in the middle third of the pitch, winning some tackles, a few interceptions, making a lot of unadventurous passes.
Given his age, his size, and that he has now had a long run of games in his 2nd year at the club, I don't see him improving much on what we've seen so far.
McTominay is different in that it is easy to see his value in terms of the goals he has scored and the headers he wins inside our penalty area. Of course he needs to improve other aspects of his game but I expect him to be a useful player for us for the next few years.