Injuries are a factor in the NFL. It's often said that once the season begins, no player is ever going to be 100% health-wise. It's a grueling game that takes a physical toll over the course of the season.
While the NFL is better at disclosing injury information and player status than other sports, it often still leaves fans and bettors with limited information until the end of the week and into the weekend. Players practice some days and miss others, or players practice in limited capacity through the week. Some players don't practice at all and end up playing.