Why would a team put a player on IR?

Being put on IR means that the player can’t perform and is “not immediately available for participation with a club”, as per the NFL. There are also different designations, such as the PUP (Physically Unable to Perform), reserve/non-football injury, and reserve/non-football illness.

How long is a player out on IR?

An unlimited number of players can return from a team’s injured reserve list, and they are eligible after missing a minimum of only three games. After three weeks, the player becomes eligible to return to practice. Once he returns to practice, the team has 21 days to place him back on the active roster.