High value does not come from performance. Your buying Q + Training rate.

Players get bad ratings because they are bad performers or because they are put in a position where they can't perform well. If an MC, DMC or GK is rarely involved and makes 1 mistake the ratings will hammer him - first question I ask is did my system (opponents system give him any opportunities) - if the answer is yes, bench him. If he's not that important to the team, either persevere and bench him or sell him - but first check if there are events that occurred that could have contributed to it such as injury and PK misses.

PS. my ST scored 2 PKs and missed 2 PKs in one game, got injured in the next and slumped - benched, subbed, cuddled then back to form but took about 11 games to get him back on form, 4x8 n 3x9 in last 7 games (mix of 7,8 & 9s before then).

The numbers 1st 7 days rated - 8.08, 2nd 7 - 7.92, 3rd - 7.10, 4th 8.08. PK misses were on Day 12, Injury Day 14 iirc

PS. Ofc it could just all be Voodoo - in my head