Defend the Space
A lot of the things you'll do in Fifa are similar to what you'll do to defend in real life. You'll rarely find a defender going full aggressive and moving out of place to tackle an attacker. That's just something that doesn't happen. You're not only guarding the ball. You're also guarding the space. If you tear off from the formation, you're breaking the integrity of your defense and leaving a space open for passes and even a goal.
Go Manual
Back in the old Fifa days, the best way to defend was to just hold down the run button and watch as the AI slowly took over and defended for you. The defending part of the game wasn't that enjoyable, so EA Sports just took that out of the game. However, in Fifa 21, it's best if the AI leaves the defending to you. Defending has definitely improved over the past few years, and even though the option for automatic defending is still there, the manual is a lot more reliable.
