My experience is that some players respond after a while of adjusting to the team or being benched a few days while others are just busts.

If I have a talented player performing that poorly I will bench him a while but if he comes back and still sucks I cut my losses. Good news is that at least in the lower levels other managers don't seem to look too hard at a players form when buying and you rarely have trouble getting back fair value unlike real life.

Form is king. If you have a less talented player who is getting nothing but 7-8-9 ratings then keep him in the lineup. Just as the most talented player you have is worthless to you if he is constantly under 7. It can be hard to let go of a $2M player at lvl 1 but it is the best move sometimes and you can use the money to get 2-3 players who perform well. This far into the season I would suggest looking on the market for 4-5* players that have had good form. You can see their stats so pay attention to them. If you find a player at a position of need with good ratings go get him, for some strange reason a lot of managers sell players in good form.

Stay away from players with no stats this late in the season unless they are 18-19 yr old nordgens (players from teams that sound like one of the 7 dwarves like AC Clappy etc)