How to deal with form problems?

[Blitzwing013]

Same problem here...

One of my best players in stars,but one of the worst in form...
At least 2 mistakes were goals; What I don't understand...in one game he wasn't bad according to the gameplay. Stopped 2 free kicks,and no weird things,but still a 5..
Should I sell him and let someone else get headaches or just bench him,power train him and let him play in friendlies?