Nothing wrong with his attitude or energy, I just think he is incredibly limited as a player. He runs a lot. That is literally the only strength of his game. He isnt a player with particularly good positioning (indeed its more the opposite, which is why he thrives when given license to just roam around the pitch... running), he contributes very little in the final third, his passing is poor*, and he is prone to sloppiness and gifting away possession.
I think and hope Fred can improve a lot under Rangnick, but right now I think he is simply too limited to merit a place in a team that wants to play any sort of progressive football. For a backs-against-the-wall situation like we expected vs Chelsea (given the teams form), or as someone to bring on to break up play and defend a lead, absolutely - but thats it.
*I have posted in more detail about Freds passing before, but the TLDR is that 'pass completion' does not tell the full picture. Fred is regularly guilty of underhitting, overhitting or misdirecting passes which either kills attacking momentum or plays the recipient into trouble. Just because a pass reaches its target, does not make it a good one. Pass completion is far too binary a statistic to be a useful metric by itself. Stats can be deceiving, it should be obvious to a viewer that Fred is not a good passer of the ball.