Good players turn poor

Two of my star midfielders who consistently played well for the past 5 seasons have suddenly decided that they can't be bothered. They have gone from averaging ratings of about 8 per game, scoring plenty of goals and being top of the league assists table to getting 4s and 5s since the start of the season. They have not scored a goal between them in the first 6 league/cup games plus 4 friendlies and have only managed one assist between them.

My team tactics have not changed, there have only been a couple of additions to the squad and I am still winning games. However, these two are offering nothing to the team which is odd. I've tried shifting them around a little but they still play poorly.

Anybody else noticed star players becoming really poor and not improving?