Injured players play badly
This will show itself in less effective passing, shooting and less effective marking against opponents. An injured talent level 7 player may only perform as a 2 for example because of his injuries.
How to identify Injured players
When looking at your team either during team selection or during the match, note the colour of their position and rating label, for example DM7. If it is white, they are fit, yellow= minor injury, orange= more serious injury. You can always pick a player to play, but the more injured he is, the worse his performance in the match will be. When a players description label is showing as orange, try to avoid picking him to play!
Healing injured players
Players will slowly recover from their injuries when they are not playing in your current match. Serious injuries (Orange player label), will take longer to recover from than minor injuries (Yellow player label). To rest players and help them recover you need to put them into reserves or as substitutes. Substitutes still have to be ready to play in the match and warm up, so injured players selected as substitutes will recover more slowly than players you have put into reserves. If a substitute plays in a match of course that also slows recovery from injuries.