As a general rule, P4 or higher and I rest the player during the regular season.
P3 depends on injury percentage, 65% one area and up is a go but I may lower him on the depth chart.
Questionable depends on the length obviously and injury part. I've seen Q0 for a concussion or Q1 or Q2 for a hand injury so if they touch the ball then no go but linemen can play.
Doubtful is a don't play period.
In the postseason, it's more complicated depending on what round and opponent. Once you hit the Conference Championship or League Title game then you have to play like there is no tomorrow.
Injuries test the depth of your roster and I try to make my roster as deep as possible everywhere but QB & kickers since they rarely get hurt. So my handling of injuries is ultra-conservative.
The real problem begins when you only have 42, 43, 44 or 45 players who aren't injured since you must have 46 active.
When that happens, I will activate the less injured players but hide them in the depth chart to limit playing time.
