Whats a defender supposed to do?

[Jeff23]

I am having a really hard time with this "defenseless" receivers hits.

[youtube]http://www.youtube.com/watch?v=O76d2yVcZaU[/youtube]


I mean what was the guy supposed to do? Whats a defender supposed to do when he reads the play, waits until the ball gets to the receiver? Let him make the reception and then let him run some yards and tackle him down or what?