Differentiating between a simple down year and an inevitable decline due to aging and diminished skills can be difficult. Veteran players with long track records of success can have a tough season, then rebound to something close to their typical performance.
Sometimes, though, that down year isn't due to just a few bad bounces or a nagging injury needing an offseason of rest. Sometimes, that past performance just isn't coming back as the player heads slowly -- or sometimes quickly -- to the end of his career. Younger players might not be as close to retirement, but an off year could be an indicator that we've already seen the player at his best.
Quite a few players had discouraging seasons in 2018, so it's worth checking in on a few of them to see if they've been able to rebound or have had a continuation of last year's troubles.